Not sure if this is a bug from recent site updates or a feature (I’m not on here as much), but I used to be able to edit topic titles, (e.g. to fix typos), but can’t seem to anymore.
(Not a big deal, mostly mentioning in case it’s a symptom of something more important being borked.)
I think what it is is that there’s a 24-hour limit on post edits. So you remember seeing it at those times.
It’s always been set that way since we started using Discourse, FWIW.
I’ve definitely edited much older posts a time or two when the original subject referenced a time period that was long past. But the originals would have been pre-Discord, so…maybe? 
I think you must be mistaken. Or I am!
I’ll see if I can find one.
I have also done this, but I believe it was when my permission level was higher, probably Regular.
Ah, here we go, I remember this one had a title (not subject, sorry) referencing a few years, but having started in 2018 I made it a little more generic:
If you’re a Regular you have the ability to edit titles, you might be remembering that.
Isn’t that what OP is talking about, editing titles?
Is there another thing called subject that is different from a title?
Aha, that’s it, I am merely a member now. 
(Hat tip to @meishijia - you called it!)
You’re right, my mistake!
Yep @bz , you must have had Regular status when you made those edits, but you lost it at some point. You need to view more topics and read more posts to get it back.
My mistake originally, now corrected for posterity! 