> Distinguishing messages as urgent vs. non-urgent (no notification) _could_ alleviate some of this issue, but it's ultimately a culture issue, not a tech issue.
I think drawing a hard distinction between culture issues and tech issues is a mistake. Tech influences culture and vice-versa.
The fact that Slack saves the full message history is great for being able to refer back to previous discussions, but the fact that the history view of every joined channel is the default interface encourages this sort of asynchronous messaging.
IRC was closer to synchronous in that you could leave channels but retain permission to rejoin them. So you ended up with a sort of split in the culture because it supported two different usage patterns; some people would use a bouncer and idle in every channel (this is basically Slack), while some people would connect and join only the channels they wanted to be in at the time (unsupported by Slack). And of course there are levels in between those two.
MUDs could be considered the option that’s fully at the latter end of that spectrum. You may go AFK and idle, but you’re only ever idling in one room, and it’s often made clear if you’re AFK or not in the interface. The result is that for synchronous chat, there’s one single place to pay attention to: the room that you’re in. (Whispers are supported but they typically show up in-line as a message.) For asynchronous chat, there’s a separate mail system (which often forwards messages to your email address as well). The two are distinct in the UI.
I think MUDs got the sync/async distinction right in a way that modern chat apps don’t.