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
For ongoing discussions about a topic, start a channel and perhaps prefix it "temp-" to indicate that it's a temporary channel.
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.