It isn't fully asynchronous if notifications are enabled. With notifications, you are more like a single cpu server that has to handle hardware interrupts for disk/network io. It stops you from working and forces context switching, only people aren't as good at context switching.
Pro-tip if you're one of those: If you look at the slack interface, there's not a lot of visual difference between two messages in a row, and a single message separated by two newlines (SHIFT-Enter if you don't know how to enter those).
It looks exactly the same to type "Hey.\n\nJust wanted to check up on the status of the de-fooification project. How's that going?" as it is to send those as two separate messages, it looks just as polite on the screen, and it's way less annoying.
Even worse is "Hey, I want to talk about something. I'll call you in 30 minutes."
"hey" [waits for a response; then a long pause and agonisingly slow typing after you've said hey back]
"Can I ask you a question?" [waits for a response] [agonisingly slow typing coupled with intermittent pauses].
Always makes me want to yell at people to just ask their damn question.
Granted, I dont think I ever stopped at 'hey-'
You don’t have colleagues who type “hey” and then wait for your reply, do you?