I have a problems with this, particularly as implemented without discussion, and also, in response to this:
“. . . so we could make some of these types of channels public but not allow everyone to post in them.”
That is definitely possible—there is one group that I have joined on slack that has an announcements channel that states “only some people are able to post to this channel”—though anyone can comment, so maybe that is not quite what is wanted.
And of course, AFAIK at least, private direct-message group conversations can be started by anyone at any time, so I guess it did not even need to be formally announced.
The more I consider, the more confused I get.