@François Dorais: Concerning the tag geometry, there are about 500 questions with that tag which do not also have a geometry-related tag: https://mathoverflow.net/questions/tagged/geometry+-dg.differential-geometry+-mg.metric-geometry+-ag.algebraic-geometry+-gt.geometric-topology
@FrançoisG.Dorais Thanks for your reply. There are many questions where I am not sure what would be suitable tags. However, I'll try at least to follow the deprecated tags and remove them from new and bumped questions - so that the tags at least do not grow.
@FrançoisG.Dorais You recently mentioned on meta: "So now is a great time to allow inconveniently compressed tag names to stretch out a bit..."
As a reaction on this I've posted an answer in order to collect suggestions for this.
Only then I realized that list of tags which have 25 characters can be found using SEDE. The query is linked in my answer, too: Tags with given length of name
So should I simply delete that answer - probably moderators can go through that list and rename the tags. (If some of them are problematic, they can be discussed on meta.)
The above query returns 38 tags. But some of them are not shortenings - the tag originally has name of length 25.