As mentioned above, our Support/Engineering teams will support you if you find a breakage caused by a new version of ConfigMgr. However, App Portal's integration with ConfigMgr is at such a basic level that it's unlikely any new version of ConfigMgr will break our integration at the SDK level. Our Engineering team is constantly running test passes as they develop the product, and they will generally test on the latest publicly available release of ConfigMgr as they run through their test passes. While the "official" ConfigMgr version support would be updated in our documentation when we release a new version of App Portal, it's likely that it would have already been tested much earlier than that.
Keep in mind that the statements here are in reference to existing App Portal functionality working with new versions of ConfigMgr. Any new features that become available in a future release of ConfigMgr would be evaluated on a case by case basis to see if it would add sufficient value to the App Portal product and added to the product backlog for future development based on value/priority.
Anything expressed here is my own view and not necessarily that of my employer, Flexera. If my reply answers a question you have raised, please click "ACCEPT AS SOLUTION".