I was a bit suprised that agn did not exist, and neither does active-galactic-nucleus, and I just created it because a search showed 72 results (as of 2021-04-15). On second thought, it is probably a good idea to ask here, what the community thinks about these tags, and in particular how big the overlap to active-galaxy is, before I start adding agn-tags to older questions.
IMHO active-galaxy contains a agn, but a question about a agn is not necessarily about a complete active-galaxy.
Now, that we are at it, should we also create active-galactic-nucleus and make it a synonym of agn?