Well I don’t know about ‘officially supported’ but NextCloud is part of the MIAB installation as you know. It is primarily intended to provide calendar and address books. Of course, NextCloud includes file management as its core feature so that is also in MIAB. I use it as a document store myself and have done so without problems.
NextCloud is updated from time to time in MIAB. I see that there are plans to update Nextcloud to 20.0.8, contacts to 3.5.1, calendar to 2.2.0 in a future release.
I see that there has been some discussion about removing NextCloud but I think the consensus was that it might be made optional but not removed as many people rely on it.
Adding other features to NextCloud is not supported as far as I know. The script which allows admin rights necessary to add features warns " Run this at your own risk. This is for testing & experimentation purpopses only. After this point you are on your own."
Other than that, it could be argued that using MIAB to store documents adds a security risk. You will want to make you own judgement on that.
There are alternatives to MIAB which support NextCloud with extended features. I use one such to provide NextCloud documents and collaborative editing. My production email however is all on MIAB
Hope this helps.