The licensing is clear that further support is not required, but I will say that I personally hold myself up to patching any and all preexisting full releases of the add-on for bugs or unintended results. If you do experience any issues, get in contact with me via the contact info below.
No. GrabDoc will forever only support stable full release branches of Blender. Add-ons should still work in higher versions than 2.91 (and sometimes even lower versions) since the API hasn't seen major changes that pose a compatibility risk, but stability still isn't guaranteed.