Totally disagree. For introverts, "everything public in slack" means that I would rather not say something than have 50 people see my thoughts/rants/silly questions in public
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.
The issue is what is best for the company. If a person is not asking or answering questions then they are not a valuable member of the team.
Keeping stuff secret and hidden does not help the project.
If your team is 50 you are probably not the only one asking the question and several people would be able to answer the question, limiting to one just annoys the one you asked if they are busy and someone else is not.