These queries cannot be considered completely reliable - since we do not have data about the past revisions of deleted posts in SEDE - but there are some queries to find tags created in the past. (Such queries are sometimes mentioned in the Tagging chatroom.) See also: Can you tell who created a tag and My very own tags.
- Tags created by a specific user
- Questions which had the given tag (including the editor who added it)
The situation with the tags fubini-tonelli-theorems is a bit more complex.
The tag fubini was created in June 2019. The tag fubini-tonelli-theorems was created in July 2020 and the tag fubini was merged into this tag in September 2020. And when the tags were merged, all occurrences of fubini were replaced by fubini-tonelli-theorems - and the tag was also changed in the revision history. (At the same time, the original tag (fubini) was removed.) The deleted questions are an exception - as you can see some deleted questions are still tagged (fubini).