I recently upgraded from Mojave to Big Sur. Time Machine continues to backup without issues, but when I try to restore anything from before the os upgrade, it is dimmed on TM’s timeline and can’t be selected.
I can however browse the TM disk directly and copy the needed items.
I also recently did the upgrade so I experimented with TM and my desktop folder. I was able to restore an old file. It took many seconds for the past file to appear. I assume the information had to be reformatted
I should not have used the desktop as many icons were moved. Fortunately not deleted