For better or worse, I think Slack’s approach of opt-in threading is effectively the default at this point. If users’ (technical or otherwise) mental model assumes that messages can get sent to a channel, but the software only allows messages to be sent to a thread, that disconnect could result in lots of unexpected behaviors.

For what it’s worth, I personally prefer Slack’s approach over Zulip’s - I think threading is a really important feature, but requiring it for every message creates excessive and unnecessary friction.