There are plenty of times when a synchronous or time sensitive communication is needed and in those cases a proper first message in slack is something like "pro

There are plenty of times when a synchronous or time sensitive communication is needed and in those cases a proper first message in slack is something like "production is down" or "we have a customer call in an hour and I need an update on issue X" as those types of messages communicate clearly that I need to drop (or wrap up) what I'm doing an attend to your needs. If you say "hello" instead you are immediately telling me that your issue isn't an emergency, but that you want me to treat it like one. I just ignore these and if the person goes on to ask their question I'll deal with it when I have time.