Okay so a thing about "unwritten rules" that people keep sharing is that people don't actually agree on these rules "everyone" allegedly agrees on.
So new folks get told they're doing it wrong because some ill - defined "we" have definitely decided on XYZ, but no one actually decided that. It's just somebody's opinion.
I can't speak for the whole fediverse, but on the wandering shop, our rules are written down. They're on the about page.
And self promotion is explicitly allowed.