A lot of this is actually just the effect of how the story comes down to us. I won;t provide any sort of cutoff year, but as a general rule of thumb, the farther back in time you go the less likely you are to be getting your information on wars from things like administrative documents and military dispatches, and more likely to be getting them from chroniclers. Chroniclers obviously have limited space, and so they often need to focus on a single narrative thread, and so "minor engagements" are often passed over. Compounding this, the chronicles we often have are not even chronicles of specific wars, but rather a much broader history, which means that even more gets eroded away.

As an example of this, take an incident in the civil wars of 69 CE, in particular the war between Otho and Vitellius. During this war, the fleet of Otho ravaged the coast of Liguria (northwest Italy) and brought a great degree of destruction to the population there, and fought several actions against forces allied to Vitellius. We know of this because it received a few paragraphs in Tacitus' Histories, which is a five chapter description of the period, and a brief mention in Agricola because the mother of the titular figure was killed in the raids. I should also note that Tacitus himself was from the neighboring region and was married to Agricola's daughter, and so had a very personal connection to the raids, which would have taken place when he was a boy. If you look at other histories of the period, such as the relevant sections of Suetonius' Twelve Caesars and Dio Cassius' Histories the raids aren't mentioned. It is worth noting that Tacitus' manuscript history is extremely weak: half of the Annales and the Histories come to us from a single manuscript.

So we would not know about the raids on Liguria if (1) one of our historians was not personally affected by them and (2) if a stray fire had obliterated the text from history. So imagine how many "minor actions" happened which did not have this luck, and thus we don't know about.