The Sangoma team does not test additional OSes outside of their own. All the manual install documents are community based and happen after someone has tested the install process on an OS and then adds the documentation.
So right now Asterisk 20-rc1 can be downloaded and used with FreePBX v16, the GUI doesn’t have anything special in it that would stop it from working with Asterisk v20. If there is an install issue with FreePBX then we would need to know what OS it is being installed on, if all the requirements have been met and what errors the person is getting. Then it can be determined if there needs to be a ticket because code needs to be fix or there is another issue (a wrong version of a dependency or missing).
I have yet to have a problem installing FreePBX v16 with Asterisk v18 on Ubuntu 18.04. I’ll be testing with Ubuntu 20 soon.