Buddhism was extremely widespread in the area prior to the arrival of Islam, and was probably the dominant religion, though virtually every other then-extant religion was represented in what is today Pashtunistan. It's not known for certain, however, what religion was dominant among the Pashtuns themselves, if they existed at that time at all. Ancient sources are relatively replete with references to 'Afghans' and 'Afghanistan,' but these are not proof of the existence of specifically Pashtuns. Afghanistan, today, is home to a large number of Iranian and Turkic ethnic groups, including Pashtuns, Hazara, Turkmen, and Tajiks, any of whom (except maybe the Hazara, who are a syncretic people) may have been the 'Afghans' of which the ancients spoke. Today, the majority of Pashtuns live in Pakistan, due to some arbitrary lines drawn by the British.
Nevertheless, after the arrival of Islam, its influence was fairly totalizing on the Pashtuns (or the people who would become the Pashtuns). The modern Pashtun way of life, Pakhtunwali, is very Islamic, and, yes, somewhat violent. To this day, it includes elements of what is known in the west as feud or vendetta.
But, the Buddhism practiced in the area did have lasting influence and did have some unique characteristics. Specifically, it had a notable influence upon the Greeks who settled there during the time of Alexander's conquests, and upon the states those Greeks founded after the fragmentation of the short-lived empire, such as Bactria and the Indo-Greek Kingdom. Indeed, it was essentially adopted as the official religion of the Hellenized elite in the area, and as a result there are, for example, sculptures of the Buddha done in the realistic Greek style, such as this one: https://upload.wikimedia.org/wikipedia/commons/e/e2/HellenisticCoupleWithBuddha.jpg. More than that, the Greeks, as they always did, heavily syncretized their religion with new ones, and translated many of their gods into Buddhist forms and translated many Buddhist figures into gods and heroes and the like.
The Indo-Greeks and Bactrians produced a huge amount of art and architecture (such as the many Yavana caves: https://upload.wikimedia.org/wikipedia/commons/2/20/Cave_17_at_Pandavleni.jpg; 'Yavana' is Sanskrit for 'Greek-speaker,' and inscriptions in these caves show that the Yavana were major sponsors of them), combining the culture of their original home with that of their new, much of which survives today in museums and on-site in central Asia. and which had an influence that outlasted the Greek kingdoms in the area, until its disappearance in approximately the 5th century AD, which was also just before the arrival of Islam, in the 7th century.
So before we begin, I think I should clear something up that while not explicit is rather implicit in the question; that somehow there Buddhism makes a group less warlike, which is not true. Buddhism, while technically holding killing and warfare as wrong in all cases, has a long history of warfare and violence. Recently, u/jimedorje gave a pretty good post about it. The myth of Buddhism as a religion that is somehow more peaceful or pacifist, often inn contrast with Islam, is a dangerous one that has been weaponized in places like Myanmar, Thailand, and Sri Lanka. Afghanistan was not more or less warlike because of religion and the people of Afghanistan were not more or less warlike because of it.
Furthermore, I think we should examine what "warlike" means and why a people are considered warlike. The idea of a group being warlike has immense implications with how they are viewed externally and is a loaded designation. It has the implication of less intelligent, unenlightened, or barbarous, used to dismiss a group in contrast to ones own. It is also not a particularly useful term, since it doesn't really tell us about the group. What does warlike here mean? Do they place emphasis on war as a means of conflict resolution, do they live in a violent place, is raiding a significant economic contributor, or none of these? So perhaps we need to examine who considers the Pashtuns as warlike and why.
The people of Afghanistan have been seen as particularly good soldiers dating back at least to Ashoka, who gave some degree of autonomy to the people of Afghanistan. However it is unclear as to whether these should be considered the ancestors of modern Pashtuns. The earliest known writings that we can clearly designate as Pashtun come from the 16th century with Pashto poetry, but the Pashtun people must have been in Afghanistan and Pakistan much earlier. If the Pashtun people are identical or related to the earlier people whose exonym was "Afghan" (more on that later) then our first verifiable reference was probably Xuanzang (玄奘) in his travelog. He calls them "fierce and impudent" and notes a resemblance to Indian languages but that it was rather different, so it was likely an Iranic or Iranicized Indo-Aryan language. In any case, it is clear both from Indian and later Persian sources that the people of Afghanistan were seen as fierce and sturdy warriors, but not necesarilly warlike.
Islamic sources continue this kind of narrative regarding Afghans until the Mongol Invasions. The Mongol period seems to have destabilized the region and upset the balance between the tribal semi-nomads and the urban elite with many turning to raiding and robbery as a way of survival. This was consistent with much of the region where the Mongols entered. However, even during this time, among the Turks and the Pashtuns, people like Babur, the first Mughal, fondly looked back on his time in the region and does not note it as especially warlike in comparison.
It is the British who first seem to have judged Pashtun society harshly, with colonial records calling them "uncivilized" and the Pashtunwali described as "a rude system of customary law" by an East India Companyman in 1809. The Anglo-Afghan wars were brutal and violent, leading to animosity on both sides. We see the narrative of the Pashtun as brutal, warlike tribesmen emerge from this period of interaction.
Broadly speaking, this narrative is not unusual and was rather common in colonialism. A major power comes in to impose its way of life, sees the locals as barbarous, and stirs animosity. Pashtun society seen through the lens of colonialism and modernity creates an incomplete and unjust picture of the society that they live in.
But back to the original question, did the Pashtun's practice Buddhism? Did it have any special features?
The region that the Pashtuns currently live in was broadly part of what we call "Greater Gandhara," this being the area of Gandharan cultural influence. Gandhara was an ancient kingdom lasting from the late Vedic period to the Achaemenid Empire who made it a satrapy. It became a contested region after the death of Alexander inhabited by Greeks, Gandharans, Persians, Bactrians, Sogdians, and "Kambojas." It was probably at this time that Buddhism began to deeply penetrate Gandharan society, especially during the reign of Ashoka the Great. Between the fall of the Mauryas and the rise of the Kushans, Buddhism appears to have become the largest organized religion in the region and hundreds of stupas and viharas were built. The Kushan period seems only to have acellerated this.
A note on the Gandharan people and Gandhari language. The Gandharans proper, that being the developers of the Gandhari language, were Indo-Aryan culturally and linguistically and formed at least the upper strata of individuals in the region. Gandhari is a middle Prakrit with conservative and Archaic features compared to other Prakrits like Ardamagadhi. It seems to have fallen out of use in the 4th or 5th century CE, perhaps due to population migrations.
Gandharan Buddhism was relatively orthodox as far as we can tell. There were significant developments in Mahayana within it, but this was a trend all over India. Perhaps the most significant feature was the Gandharan school of art which probably created the Buddha image. Under influence from the Greeks, Gandharan Buddhists began to create and formalize the Buddha image and contribute to Buddhist iconography. The Buddha was iconographicaly identified with Apollo, the Bodhisattva with kings and princes, and Vajrapani with Herakles. This doesn't mean they were syncretized beyond that though. Certainly Indo-Greeks who adopted Buddhism, which appear to have been a fair few, layered it lightly over their original religion, but understood it as Indian. The Gandharan school of art reached its zenith in the 3rd or early 4th century before declining.
Similarly, Gandhara was an important crossroads for Buddhism and many of the early texts that went into China were written in Gandhari and came in via the Tarim Basin. The schools most identified with Gandhara were the Dharmaguptaka and the Kashipiyas, later the Sarvastivadas. Buddhism appears to have declined in the late 4th and early 5th century with the advent of the Alchon Huns, with Chinese pilgrims commenting on the destruction of some areas and the relative disuse of some viharas and temples. While some have questioned the role of the Alchon Huns in the decline of Buddhism, noting that a few early kings appear on a stupa as donors, the archeological record appears to show a rapid drop in artifacts dating from the reign of Mirakula, who was the supposed major persecutor, so I will say it is at least plausible. Hinduism appears to have been the largest religion with Buddhism being a significant minority religion until the 11th or 12th century in the region.
However something to consider is that, like Sogdia, Gandhara lay at the crossroads between major regions and was a very cosmopolitan place. Zoroastrianism, Buddhism, various Hinduisms, and other faiths were all followed by locals in the region and played a role in the cultural framework. While Buddhism may not have had any "special features" that made it doctrinally distinct, Buddhism does not require one to stop worshiping local dieties and the local popular religion likely incorperrated a great variety of distinct cults and influences.
Because Buddhism was layered over this, when the monks went away and it lost institutional support, it likely faded into the background and worship focused on the local cults, making the penetration of Islam easier, though it appears to have been quite slow initially as the Afghans are mentioned in as Hindus in al-Biruni's work during the 11th century. The formalization of Pashtun identity appears to have been during the late 16th and 17th centuries as well as their creation of a common origin to facilitate a common unity through an ancestor, indicating that the Pashtunwali and tribal laws were in some force during this time. It is likely that Islam, while deeply affecting Pashtun society, did not radically change tribal customs.
