I was not able to find a discussion on meta where this was decided, but I know that this tag was not used. Since 2013 it had tag-excerpt saying: "Deprecated; please use a more specific tag." (Another example of deprecated tag is geometry - at least according to the tag-excerpt.) The tag (topology) has been completely removed in 2014.
What should be done about the topology tag if it reappears again, like it did now? Should we try to do something to prevent it from being created again?
- Should we simply rely on the fact that many users of this site know that this tag is deprecated and if it is created again, some users will remove the tag from the questions (and in the next 24 hours it will be removed from the site)?
- Should some reasonable tag-synonym be created, for example, topology $\to$ gn.general-topology? (This would prevent users from creating this tag as a new tag; if somebody will use it, the tag will automatically be changed to the master tag. The disadvantage is that some users might not notice that the tag they typed in was changed to the synonym.)
- Should the moderators request the SE team to blacklist this tag? (The obvious disadvantage is that users trying to use this tag might wonder why the tag called topology is not allowed. However, it is possible to customize the messages shown when a user tries to make a post containing some blacklisted input, see here and here.)
EDIT: Now the topology tag has been already removed from all questions. So in 24 hours the tag should disappear completely, unless it is used in another question.
But the question whether something should be done to prevent this tag from being creating again remains.