I can be away for a few days, catch up quickly, and add a comment to a conversation that ended long ago and know that the participants will actually see it. All thanks to threads.
This is obviously a hard problem to solve without just making it back into email. Muting apps that spam notifications for someone else’s benefit would be a great start. Not allowing bots to spam new threads at you and cleaning up old threads automatically would also be great—having a ton of old threads brings the client to its knees on my dual core laptop.
Slack just sucks for anything you need to find again, IMHO, it should be treated like it's ephemeral. The logs are worth keeping as leverage against employees of course.