5:01 AM
It seems that in July 2014 the tag synonym $\to$ was pending, but already with score 3: web.archive.org/web/20140708120313/http://… In April 2015 the synonym already existed: web.archive.org/web/20150414155234/http://…
Previous discussion on this:

Mar 8 '17 at 7:41, 1 day 11 hours total – 23 messages, 2 users, 0 stars

Bookmarked 4 mins ago by Martin Sleziak

Deleting the tag (conditional-probability) was a most unwise decision (whose rationalizations above do not hold water and make for a rather odd reading). People, just grab any modern probability theory textbook and tick the chapters in it on this very subject... Was the deletion acted upon by two users in all, if I read correctly? — Did Mar 8 '17 at 7:32
> Users with more than 2500 reputation and a total answer score of 5 or more on the tag, can suggest tag synonyms. Users with a total answer score (total upvotes minus total downvotes) of 5 or more on the tag, can vote for tag synonyms. Suggestions will be automatically approved when they reach a score of 4, and automatically deleted when they reach a score of -2.
@Did As far as I can tell, the tag synonym got approved by the above process. Which mean one user to suggest the synonym and 4 more users to vote for the synonym.
AFAIK when a moderator votes for a tag-synonym, it is approved immediately; the same as with close/reopen/delete votes.
But in this case, it seems that it was most likely approved by regular users. (As you can see, at some point it was at score of 3, so at least until that point only regular users voted.)

5:36 AM
I suppose when you talk about deletion you mean creating tag-synonym.
I am not sure who you mean by two users. Did you mean to say that when the synonym was suggested it was after a discussion only between two users? (As mentioned above, at that point other users to vote for the synonym were still needed.)
My question on meta that is (to some extent) related: Is there a way to monitor for “bad” tag synonyms?

2 hours later…
8:01 AM
@MartinSleziak Duly noted, thanks.