@quid, I finally took a look at the tag 'topology'. I apologize for taking so long. The tag 'topology' is a mess, so I am not completely sure. Nevertheless, I would think that the best line of attack is the one you suggested. Retag all (or most of) the questions which do not have a top level tag, and perhaps also (some of) the ones whose existing top level tag does not relate to topology. Then merge the tag 'topology' with 'tag-removed'.

What do you think?

By the way, @quid and others. Do you happen to know what are the precise criteria under which single-use tags are deleted? I could not easily find a reference. Thanks.

@RicardoAndrade: I think after 6 months - meta.stackoverflow.com/questions/48417/…

Thanks, @François. That thread is exactly what I was looking for.

@RicardoAndrade thanks! I think I will start like this then. I am not sure how it will progress. But let us see.

@quid, good! I will chip in whenever I can.

In any case, I will monitor the front page to avoid having new questions added to the tag 'topology'.