Just throwing in some anecdata but this is how it has been used in the place I've worked at since the pandemic started.

Prior to remote work being basically the norm these tools were used more like traditional real-time chat in my personal experience (I'm sure it varies even still based on particular workplaces), but now its generally async with the possibility (but not expectation) of being synchronous if you happen to catch someone randomly available to be bothered at the specific time you send the message.