Hot take: slack-style threads are bad for long-form discussion. They encourage forking, when what you really want is everyone to be on the same page. Traditional forum-style threads, or "channels" on a chat app, is optimal for discourse, as it encourages continuous convergence and integration rather than splintering. Hiding tangents serves to lower the quality of conversation.
Optimal within the constraints of real time chat IMHO is slow-mode (5 to 15 seconds, or possibly greater) in a single channel.
IME tangents, while possibly fruitful, just lead people to skip the 30+ messages and so doesn't become helpful.
Hot take: make threading an option. If you don't want them, don't use them. If you want them, then they are available. This is clearly a personal difference with no right or wrong answer. So the solution is options.
Optimal within the constraints of real time chat IMHO is slow-mode (5 to 15 seconds, or possibly greater) in a single channel.