If it's off-topic, IRC's ancient solution has always been "Take it to #channel-cafe" or whatever, perhaps a topic-specific channel. Just /join it, if it doesn't exist it gets created. Or perhaps have a 1-1. But because this affects everyone, it's easy to make it the culture to not spam irrelevant stuff in the main channel, making it a non-problem that threads could alleviate.
In practice you're going to get violators, but that happens with Slack as well. In part because Slack's thread UI sucks so occasionally you'll wake up and see a couple people in after-hours last night who just didn't care and filled up 50-100 messages on the unthreaded channel, forcing people to skim through it or if they were foolish enough to leave notifications on after-hours to get all these pings.
One nice thing basically all modern chat systems have is easy group chat, so you don't have to create a new room, but rooms are nice too. e.g. any time we had a customer case requiring a few people to look into it, we'd just create a slack channel for it instead of using threads on the team channel. After a while someone would archive the channel.
Then there are people who use threads and occasionally click the "send to whole room" option which really screws up notifications. You're looking at it right there in the main team chat, but you have to open the thread or your notifications to dismiss the notification.
Next time I'm forced to use Slack I'll use Ripcord. You know what UI I miss most and would help Slack/Discord/Element tremendously? Tabs.