What's with this "!rant" thing at the start of rants? Is it some ettiquette thing when you mistakenly mis-tag, but can't change the tag, so you have to add this to say "genuine mistake"?

We didn't have categories so we prepended the "rant" with !rant to indicate that the post wasn't actually a rant and it was usually instead something positive.

It's from before the rant classification system. Also !rant generally refers to positive stuff/stories since the rant category is for stories as well but it's weird if you have a positive thing to say (promotion etc) but it's tagged as rant.

I guess it's still not an outdated practice though since the rant could be a story.

Long ago, 2016-2017 or something like that, everything was classified as rants. There was no joke/meme, nothing of the sort. Just pure rants, but even back then already a lot of r/programmerhumor (re)posts that eventually started to become a problem for a small amount of users, including myself. So there was a long period in which such reposts were downvoted and flamed and such, because they were perceived as a cheap way to get a lot of upvotes whereas the platform was meant for ranting. That continued until finally the post categories became a thing, as well as filters for them. So memes are still on the platform, but at least people could choose to hide the stuff they didn't want to see.

Anyway back to those early days. Because everything was a rant and back then already people just wanted to talk about life in general, it was difficult to keep such posts separated. So in order to tell people reading the feed that this was not dev-related (!dev) or not a rant (!rant), these tags became sort of an unwritten agreed upon way to reflect that so that people knew straight from the feed what to open or scroll past. Now of course that's all in categories. Life posts for example got into random or are sometimes still posted as rants with the !rant and/or !dev tags.