The official line is “if you want to run Big Sur you MUST use a version newer than 2019r3.2” – at least for desktop apps (web apps arent affected in the same way)
But you CAN actually get 2019r1.1 to work for desktop apps on Big Sur
Here is what I have done to be able to run Xojo 2019r1.1 on Big Sur (Intel) – and to successfully DebugRun and Build Desktop applications:
Replace these files in the Xojo 2019r1.1 application bundle with those from 2019r3.2:
Contents/Resources/SDKStubs/macosx10.14/System/Library/Frameworks/AppKit.framework/AppKit.tbd
Contents/Resources/SDKStubs/macosx10.14/System/Library/Frameworks/AppKit.framework/Versions/C/AppKit.tbd