I don’t think this actually corrects anything Someone_Evil has already said, but since there was some uncertainty:
D&D 3.5e
“Aegis” isn’t any particular thing. The word almost-certainly shows up here and there, but it’s not something tag-worthy.
Pathfinder 1e
Aegis is a psionic character class published by Dreamscarred Press, a third-party publisher (who, disclaimer if that’s relevant here, I’ve worked for). As a class, particularly one that has unique mechanics, it makes perfect sense to have a tag for it. Once upon a time, DSP considered possibly publishing work in D&D 5e, or Pathfinder 2e, and that might have eventually included the aegis, but as far as I know, DSP is pretty much defunct at this point.
D&D 4e
One of the core class features of the swordmage class is their “aegis,” and that’s where they choose which sort of swordmage they want to be—they have a choice of “aegis of assault,” “of shielding,” and “of ensnaring.” The aegis is a key part of what makes a swordmage a swordmage, and which one you have has pretty significant ramifications for how you do what you do. But swordmage can and does easily cover questions about them.
D&D 5e
Like 3.5e, 5e doesn’t really have anything “special” for the word “aegis,” it’s just a word that gets used sometimes. Nothing that requires tagging.
Ars Magicka
Completely unfamiliar with this, can’t add anything to what Someone_Evil said.
Opinion
It looks like aegis refers pretty much solely to the Pathfinder 1e class; that’s the only place it’s used, and it seems like it’s the only place it should be used. The Ars Magicka questions look like maybe they could have used the tag, but I lean towards deferring to the people who asked those questions (who surely know more about the system than I), and assuming those questions are fine without it.
Thus, the questions we should be concerned with missing the tag are those with pathfinder-1e and mention the aegis, but without aegis. I.e., this search: [pathfinder-1e] aegis -[aegis]
. Seems there are 60 questions in there.
One issue, though: the aegis is a psionic class. That also implies that questions about the class should also have psionics—which is a little awkward. Because psionics are third-party in Pathfinder, we have used the tag to “section off” that content—which is probably helpful to a lot of readers; it is for me—but it turns the tag into something of a meta tag. We include it because we’re discussing DSP content from one of those books, whether or not we’re actually asking about the psionic mechanics, or even asking about psionic material in the first place. That implies we should also maybe take a look through [pathfinder-1e] aegis -[psionics]
, which has 27 results, and add psionics to several of those. Or we should rethink how we use psionics, at least within pathfinder-1e (in other systems where psionics is part of the official system, it doesn’t have the same problems).
Either way, for both searches, we must still be careful about false positives. I see several of those questions in both searches where there is a mention that the character is an aegis, but that information is entirely tangential to the question, which has little to nothing to do with the aegis’s mechanics or even any psionic material at all, even when using the expansive definition often used in pathfinder-1e.