I was suprised that there is yet no necromancy tag on WorldBuilding. Please, consider creating it on WorldBuilding, as users can be seeking questions specifically about necromancy for their worldbuilding.
Tags serve more purposes than just filtering
And because of that, it's not valuable to have a tag for every aspect of everything. I'm not specifically objecting to a necromancy tag. I'm merely pointing out that it's not enough that someone might want to filter questions specifically for necromancy.
Tags are a funny thing. They kinda scope the question — but here on Worldbuilding they primarily scope the answers. In other words, a necromancy tag is more valuable for helping respondents know that you want answers that specifically reflect necromancy, and not, for example, the use of technology to reanimate the dead.
But as @L.Dutch points out, we already have a magic tag, and that tag would just as adequately scope the answers to the question. Thus, (and for example), if you asked the question, "given the rules of my world, how could I reanimate the dead?" the magic and necromancy tags would have an identical effect.
Thus, necromancy would inevitably end up as a synonym to magic. Maybe it's worth proposing as a synonym. But I don't see its value as an independent tag. And it's very much possible to have too many tags (we already have scads of tags that have ten or fewer questions associated with them, proving that the ability to filter doesn't have that much value).