I suppose SmarterTools wants as soon as possible to only have one sourcecode/branch to maintain/edit, so their aim could  be to release MAPI as a public/stable release very soon. Or they could release their new code-branch as "stable" and with just keeping MAPI in its beta-state. Due to the massive number of entries/comments just concerning MAPI-protocol it should be clear that there still need some issues being addressed+solved.

There was a beta category during the first month of the "closed beta" - but there it was very quiet, was too early to address any customers for beta testing as Derek already admitted.

I agree that kind of a "MAPI public beta" group/category at community could make sense.
Or - in case of having a MAPI release soon - just a "MAPI-protocol"-category (without beta and without public in the name).

They  had a Beta forum but they deleted it and all the MAPI related problem posts.