Occasionally some tags on MathOverflow are deprecated - it is decided that they should no longer be used.

If a deprecated tag is only used in a few questions, then it can be removed manually - the amount of bumping is not so big that it would cause problems.

From what I understood, moderators can also merge tags. Sometimes merging a tag into is used to effectively remove some tag without bumping. (Although the questions tagged originally with the deprecated tag will still have (tag-removed) tag, at least the deprecated tag is no longer offered to users asking new questions.) So this is another possibility how a tag can be removed.1

I have seen that some deprecated tags have been not replaced by (tag-removed).2 I assume that this is in situations where it is preferable to remove the tag manually, because replacement by (tag-removed) could lead to many questions which are mistagged.

Ideally, the deprecated tags are going to be eventually removed. (Although this might not be realistic if the tag contains too many questions.) I understand that going through many old questions and editing them simply to remove a tag would not be ideal. But sometimes a deprecated tag is used in a new question. Or an older question is bumped for other reasons.3

What is correct way to handle such posts? Is removing deprecated tag - perhaps with tag summary or comment explaining why - reasonable way to go? Or should this only be done in situations where the editor can think of better tags to replace deprecated one? (Probably we should at least try to not use deprecated tags in new questions - so that the number of questions tagged by deprecated tags at least does not grow.)

TL;DR: What is the correct way to handle (recently active) questions with deprecated tags? Should deprecated tags be simply removed from such question?

1See also here and here.

2Examples I am aware of are , , and . The tag (abstract-algebra) is mentioned in the tag cleanup thread. So is (discrete-mathematics) tag, see this answer. The tag (geometry) is listed among tags which might have been created accidentally when a hyphenated tag is entered incorrectly in this discussion on tea. (Thanks to François G. Dorais for pointing me to this discussion.) For some details on the (topology) tag, see: Reappearance of (topology) tag. In three cases, the fact that the tag is deprecated is explicitly mentioned in the (current revision of the) tag-excerpt: abstract-algebra, discrete-mathematics, geometry.

3For example, this post announced that several questions tagged are very likely to be bumped in connection with updating image links: Apologies re image links update => bumping.

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    $\begingroup$ I think that silently changing e.g., geometry to mg.metric geometry---as you have been doing---is the best approach, i.e., no "comment explaining why." I don't think that mindlessly removing a tag without replacement would be wise, which rules out an automated process. So I am inclined to the incremental-improvement approach. $\endgroup$ Commented May 28, 2017 at 12:52
  • $\begingroup$ I was not really doing the tag edits silently. I always mentioned in edit summary that the tag is deprecated. (Well, it's possible that occasionally I've forgotten, but I definitely try to include that information most times. Occasionally I have even asked in chat for an advice about appropriate tags.) $\endgroup$ Commented May 28, 2017 at 13:16
  • $\begingroup$ Yes, leaving a comment concerning your edit is appropriate, in the "edit summary." But I think a comment to the post itself would be distracting; that's what I meant by "silently." Sorry for not making the distinction. $\endgroup$ Commented May 28, 2017 at 13:55


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