ActivityPub? Just because people use it as a Twitter clone doesn't mean you can't run mailing list style content on top of it. It would be nice to not use Mastodon-isms if you're trying to go about doing something like that. However it's easy with Mastodon-isms too. Have a bot listen to mentions, use any sort of moderation/accept queue to accept questions, then Reblog the ones you accept.

ATProto would fit most of the bill too here but AP is self-hostable and contained in a way that ATP isn't. AP is also standardized and has gone through standards bodies.