Forum posts should never include sensitive information (and never should have, and the forum guidelines said never to mark forum posts private…I just never figured out how to disallow it in code), and there is no plan to add a private status for forum posts.
The support tracker for Pro customers is still up and will remain up until it is replaced with some other support ticket system. As you note, support tickets are private by default (but can be made non-private when appropriate). The new forum does include the ability to mark questions “solved” by a specified reply (which is great). It’d probably be possible to stretch Discourse to act as a support ticket tracker, too, but for now I’m assessing some other tools, and will settle on something relatively quickly and switch to it.
Also keep in mind that we (Virtualmin staff) primarily check tickets first and then spend time on forums if we have some to spare, so you’re going to probably get a faster answer to questions in the Pro support tracker.
So…I guess to sum up: Don’t use the forum for Pro support tickets (the Pro ticket tracker is still up), especially if you need to include sensitive information, but we know the existing ticket tracker kinda sucks and it will be replaced, ASAP.