So, as usual, the problem of answering this question is in the definition. What is a "pagan?"

The problem is that though Romans had an understanding of "traditional Roman religion", Christians' understanding of paganism was more akin to anything that "wasn't Christian." There was significant scope for religious practices that could exist within this middle realm of "not traditional Roman religion" and "not Christian." Simultaneously, you have the problem of the association by Christians of anything "Greek" as pagan, so you could have people who considered themselves Christians, but were big fans of ancient Greek literature, who weren't considered Christians by hard-line bishops (whose texts most survive Byzantine antiquity).

So with this problematization set down, lets move forward.

The "official" answer is paganism survived until the closing of the (neo)Platonic Academy in Athens by Justinian. Supposedly without a center of focus to maintain pre-Christian philosophy and tradition, "paganism" as we know it would die out, taken over by Christianity.

The more scholarly answer, is that "paganism" and aspects of pre-Christian religion in the Roman Empire were co-opted into Christianity, and existed as a middle ground of disapproved non-orthodox practices, some of which might later be deemed "heretical."

To look at this from a categorization perspective, the difference between the two lines of thought is that one presumes an either or polarity. The former suggesting that one could ONLY ever be a pagan or a Christian, and that one MUST have had a definitive break in the turning from one to the other. Whereas the latter presumes a significant (if not complete) shade of muddy gray between the two, until one side emerged stronger out of the fog of identity to adhere to a clear pole.

Consider too, that the Roman pantheon, as applied to its parts of the empire, was itself a recategorization by the Romans to claim that whatever local god a province was worshipping, whatever aspect was closest to the Roman god, that was the god. So if you're backwater region had a god of war, even if it was significantly different in personality from the Roman god Mars, the Romans would go "eeeh... we'll just call him Mars." This applied to Greek and Roman gods too. They actually had significant differences. I'm not a specialist in ancient religion but I know that Roman Fortuna and Greek Tyche had different origin stories and personalities in the pre-imperial days that were fused more closely, but never fully together even by the late empire.

So with that said, there were clearly people in the rural parts of the empire, from Gaul to Anatolia well into the 7th and 8th century that worshipped their local and traditional deities. These may or may not have belonged to the official Roman pantheon, but we can still consider them "pagans" from the Christian perspective because they may not have considered themselves Christians.

But then we get into the problems of the cult of the martyrs. In many ways, the cult of the martyrs was a means of co-opting the local deity phenomenon, providing for local saints where Jesus and God on high would be insufficient for local religious needs. I'm not well versed enough to know if there are any occasions where local deities are carte blanched turned into local martyrs, but I wouldn't be surprised if this happened on a small scale, enough to stay out of the attention of orthodox "authorities" but big enough to keep local practices as is. Some scholars have argued this is in fact the origin of a lot of claims of later heresies, that these adapted "local Christian" beliefs and practices were what the Catholic church attempted to stamp out.

Going backward away from the local deity problem to the learned individuals of Athens, because Christian bishops considered almost all of Greek learning as "pagan", you had situations where individuals who were Christians, wrote "pagan" texts because they had an interest in stylizing their writings classically. After all, even as Christians, the traditions of Greek literature were well known as going back all the way to the foundations of the civilization. Homer itself was what kids were taught, even as Christians, about their own language. You see this in the defense by Christian teachers against Julian's rescript against Christians, saying that it is not only "Hellenes" who own the greek language. So because of this, you can't trust that everyone at the University of Athens was a "pagan" just because they studied pre-Christian Greek philosophy, literature, rhetoric, and history.

All of this going back to the real source of the problem of differentiation of Christian from Pagan: the determinations which survive are almost always, ONLY made by the most fanatical Christian bishops, who had an interest in seeing pagans everywhere. We know far less about what each individual may actually have believed, so we should consider, when determining such thing as who was a last pagan, that they may have considered themselves partly Christian. And no, we can't necessarily trust what they state they believed either, because there was that problem of classical stylization.

Which is my ultimate point. You can't really tell who was a "last pagan" because there is a period between Constantine and the Arab conquests, where the fight between Christianity and traditional Roman religion produced nothing but a grey slightly favoring Christianity. Only once the distinction stopped mattering, after the Arab Conquests, could Christianity emerge as the victor.