Context does matter here. A catch-all "rule" doesn't make sense. In some channels I just type my update. Those channels don't have Hello or similar. We usually

Context does matter here. A catch-all "rule" doesn't make sense. In some channels I just type my update. Those channels don't have Hello or similar. We usually never actually leave them. We're always on. Even if we go offline we're deemed "will connect later". The message will get to wherever eventually. Especially PMs to a particular person. In other channels, especially those Question/Answer channels... often the Hello is to see if anyone is around. If no one is around and the question is time sensitive, why bother asking it? Other channels work across time. Ask your question now and a person may later answer. Even later still you will get the answer. No one is offended by delays. Its expected. In still other channels, a "hello" is mandatory. Who are you? Why are you here? Did you just barge in and start asking questions or join the chat? Introduce yourself. Different context. Different rules. Synchronous vs Asynchronous. Different expectations.