I'm having the same issue with Package Control in Sublime Text 3 on Big Sur. There's an error in the console:
Package Control.package_control.deps.oscrypto.errors.LibraryNotFoundError: The CoreFoundation library could not be found Plugins loaded
I suspect that Big Sur moved or removed something in CoreFoundation that Package Control relies upon. Hopefully Sublime Text will be updated before the release of Big Sur in the fall.
As for the bold issue, I'm not having any issues applying bold text in apps with Cmd-B. Works just as it should (of course, I'm on beta 2 so maybe it was a bug in the earlier beta you experienced).
So Boss recebeu reputação de diegomachado em [macOS Big Sur] Problemas com Sublime Text 3 & CMD+B