@ToddTrimble Well, I suppose if score +6 is sufficient to consider it as community consensus, now it's up to the moderators.
Of course, we could also remove it manually, but that would mean quite a lot of bumped questions.
I guess it depends a bit on what you plan to do.
If you want to merge the tag into perverse-sheaves, then the questions that do not belong there should be retagged manually first.
If you simply are going to remove all occurrences of decomposition-theorem, then probably not much manual retagging is needed as a preparation for that.
Well, in that case it can be indeed only done by moderators.
If some manual retaggings were needed, I guess some regular users might be willing to help. (In particular, we could ask in Homotopy chatroom, since this is topic close to the users who frequent that room.)
However if it is going to be either burninated or merged into tag-removed, then this is something where regular users cannot help in other way than make suggestions.