There's also such a thing as oversharing context with people that are already burdened with a lot of context related to their own tasks. Splitting off into private conversations helps keep irrelevant members from context switching.
From my experience working remotely for 2 years, this can be accomplished by starting a thread with a descript title and posting the body (with the context) inside the thread. Just like reading across a bunch of article headlines on HN.
For ongoing discussions about a topic, start a channel and perhaps prefix it "temp-" to indicate that it's a temporary channel.