"halo" is already taken as a copyright tag. So "angel" to describe a halo has to do.
It's not only Blue Archive, but Cirilla of Azur Lane that's tagged this way.

Years ago I understand yande.re pared tags down to the bone. I suppose it was because of the numerous artbooks being uploaded. No one wanted to tag 100 pages like Danbooru.

So solve one problem, create another. I don't think it's mandatory unless someone in admin says otherwise.

But least-ways we don't have to come up with ten general tags before I'm allowed to upload, so there's that.