There’s a great debate about what versions of Xojo work on Big Sur.
IF you want to build DESKTOP apps you must use 2019r3.2 or later. Even an empty project fails to build properly.
They rely on a framework that Apple has removed and so the linking of them will fail.
Linking Executable ld: file not found: /System/Library/PrivateFrameworks/UIFoundation.framework/Versions/A/UIFoundation for architecture x86_64
Quite literally this framework does not exist – its got nothing to do with code signing etc
IF you are NOT using Xojo for desktop projects then you may be able to continue using old versions. I’ve successfully used 2017r3 and 2018r4 on Big Sur for Web projects.
I have to admit I have NOT tested console apps.
If you have it would be nice if you posted your results in the comments