Same at my company: Top level comments initiate a thread, and all responses must be made as comments within the thread.

If I had my way, I'd take it a step further and require that Slack be treated like a message board, not a chatroom. If you have something to say, then say it all at once in one cohesive paragraph or set of paragraphs -- just as we do here. I do not want to be pinged multiple times in rapid succession while someone sends every thought they have in a stream-of-consciousness fashion, e.g.:

>> hi

>> can you help me with a problem?

>> i need to do x with y but i'm not sure about how to handle z

>> oh wait

>> nvm

>> figured it out

Awful.

I'm not saying email is perfect or always works well, it just seems to naturally work better more often in some ways.

> The way we use slack is start a top level topic and group all related discussion in a thread.

My company has not been disciplined about that, and even when threads are used new ones get started to continue the same topic relatively frequently.