I will never understand people who write "Hello" or "Hi Joe" in Slack and then just wait. It has to be some form of mental illness.
It's why I don't use IM and just block myself off on those platforms. If it's not meaningful enough to write in an email then it's not meaningful enough for me to read.
At least I know slack/teams messages are meant for me. If it's important, don't send me an email about it, ping me on one of those.