Now, seriously, how do we convince people to go back to less synchronous forms of communication such as e-mail?
I just mute notifications for all but the most important channels, and sometimes even mute those while working. That way you only get a notification for @everyone-type mentions, personal mentions, and personal messages, which is not unlike email.
I think it's unhealthy to expect something different.
Slack is the remote work equivalent of "butts in chair" management. If you need information, outline what you need and email the person; allow them time to respond. Document that in a knowledge base if the data must persist and be disseminated. If you need to hold a meeting, hop on Zoom and record it, while you have a participant document the output. If you need something immediately (and it better be important), you can call or IM.
EDIT: @frogpelt said it better.
Just like HTTP.
> having their own 'standards'
What are you trying to send? HTML spam? Text mails work everywhere, the only annoyance you get is top/bottom quoting.
> huge spam / reputation issue
That's usually a problem admins deal with, not the users.
People don't expect you to reply immediately to email.
Edit (from 3 minutes later): maybe.