Unfortunately its a tricky UX problem that I haven't seen anyone master. Threads work well in Slack when you have a team culture that uses them consistently -- the channel sets the topic and threads keep individual discussions focused.But that requires a lot of effort in culture building which most people won't or can't do. And its still not a silver bullet, just an improvement. Taste's vary of course.

I like Slack's threads. I'm in a couple of Slacks which use them quite effectively.

I'm not quite sure how though, it just happened.