Done (suggested Feb. 2016, done March 2016)
I propose to deprecate abstract-algebra. It seems redundant with other tags, very broad, and the usage seems inconsistent.
I feel it is comparable to the meanwhile removed topology and the deprecated geometry.
(Explication on terminology: To deprecate a tag is not a technical measure. What it means is that the tag-description is changed to say this tag should no longer be used. Over time it could then be slowly phased out via retagging.)
Not (yet?) done (YCor Feb 01, 2020): I'd suggest to blacklist and burninate abstract-algebra. This tag exists in about 270 questions, including about 15 in the last 2 years (probably it was written many more times in spite of being deprecated, but has been often replaced or removed by users like me or Martin Sleziak. Actually, there was about 17 questions with the only tag abstract-algebra, and I've systematically retagged these questions, so at this time there should be none. Also I've browsed this list and questions almost at least another reasonable tag (AlternativelyI've retagged a few exceptions, I'd suggest to burninate it although I preferwhen the first option.other tags were meta/ non-specific/ narrow).