Adobe Flash

Big Sur has nothing to do with it.

Adobe completely killed Flash.

The announcement was made by Adobe in the summer of 2017, and all support for Flash ended as of 12/31/2020. All Adobe Flash components, both the viewer and server side components ceased functioning a few weeks later.

No. As I said, flash is completely dead.

If web developers have not moved on and re-coded their content in HTML5, their content is effectively dead.