For abstract-algebra, I think it's a good idea to first retag all the questions that don't have any of the obvious replacements: ra.rings-and-algebras, ac.commutative-algebra, gr.group-theory. That leaves a little over 200 questions...

Similarly we have 300 questions in geometry tag without having another geometry-related tag:

Only now I noticed that discrete-mathematics is deprecated based on the suggestion here. It is also mentioned in the tag-info.

discrete-mathematics tag contains 195 questions at the moment.

@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.

More-or-less along the lines of what I mentioned on meta: What is the correct way to handle questions with deprecated tags?.

@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.

However, they might be also tags which are shortened but do not use all 25 chars.

Going through all tags with length at least 20 would be much more work.

infinite-dim-manifolds would be an example of a shortened tag with less than 25 characters.

BTW on math.SE there already are quite a few "long tags". Here is a bit of transcript from Math Mods' Office.