Having it closed and only replying when I get a notification email for unread messages works quite well for me (if you know how to limit your email checking). Same as donkeyd's comment I've communicated this to my clients and they've adjusted and it's a non-problem.

Seems like they let you turn off all notifications:

https://imgur.com/a/nLePu

I'm fairly sure you can set do-not-disturb times as well.

When you open preferences, Notifications is the top pane. Literally the top notification option is "Notify me about..." and an option is Nothing. In what sense can't you stop notifications?

incorrect!

/dnd, per-channel notif muting, and "away" status combine for reasonably fine-grained control

I only use Slack's web client and I explicitly don't give it the ability to raise desk-top notifications.