Subject line says it, really. This is available only on iOS 11 or higher. However, it would have the advantage that Beorg could then use every sync service that has a Files integration (such as OneDrive, Google Drive, Amazon Drive, Boxcryptor) and that it's by now kind of the standard way to access files under iOS.
I've long wanted to get this implemented - and quite a few users have requested. I need the bite the bullet and get started :) Saying that I don't yet have an ETA for this - with schools closed in the UK for the foreseeable future I'm not getting as much time as I would like to get some of these bigger features knocked on the head.
I've started looking at this. I have though found that apps such as Google Drive and Dropbox don't allow the user to choose a folder so this feature might be more limited than I thought. There are however many apps that will work with this such as Working Copy - however not a given that an app which should support this (such as the ones you mentioned) that actually implemented that support.
If there are file provider apps that don't allow access to whole folders, it's a problem of the particular app. Being able to pull in the whole folder and see new files automatically is a "nice to have", but I think having file-by-file support on those more limited apps is still better than nothing, moreover since there are, as you said, many others that do have folder support.
That is something worth me investigating and could work nicely for a number of use cases. You could point beorg at individual org files that you want to sync with. A disadvantage would be that you couldn't create new files from within beorg. However for users that want to only use beorg with a small number of files it could work quite nicely. I shall give this some more thought.