It gets used online for things, and there are whole communities devoted to shitposting, but I can't find a clear set of rules for something to count as a shitpost. I remember querying whether a post on a shitposting community was witty enough to be a shitpost rather than just a shitty post, but of course not all the responses to that were terribly helpful!
I know it's a 45 minute Youtube video, but I love Tom Nicholas and all his stuff is fascinating and worth watching. Check this out as he does a sincere deep dive on it to get an honest answer and it's pretty enlightening! It's actually evolved even in the short time since its inception on the internet.