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!
Thanks for asking this, OP. I’ve been wondering this same thing for awhile with no real results when I search.