Turn off the friggin' notifications if you don't like them. (I don't understand why anybody leaves notifications turned on on any app, tbh -- I turn everything off the first time I see it.)
Slack warns you "are you sure you want to notify all these people" or "it's outside this person's hours"... If they have Notifications turned on, that's their problem, especially if they were silly enough to setup Slack on their phone with notifications.
If someone needs my attention instantly, they can call me. (and then leave a voicemail, because I keep my phone on silent too ;)
Turn off the friggin' notifications if you don't like them. (I don't understand why anybody leaves notifications turned on on any app, tbh -- I turn everything off the first time I see it.)