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!
It’s posts that are a step up from piss-posting, but below regular unmodified posting.