We finally ditched Slack and went back to email. Everyone (25+) has email, knows how to use it, and gets them instantly on their smartphones. We have a rule that if it can't be written in email, then to have a phone or real-life conversation.

It's been a couple weeks since we made the change, people were very skeptical, but now everyone feels a lot less stressed-out and engineering is overjoyed and its been a boon to productivity. Turns out a lot of time was wasted just chatting and being interrupted.

I'd be very reluctant to use a team chat service ever again.