Currently, MacVim still links against the Carbon framework, which apparently still works under Big Sur. However, the API has been deprecated for a decade, and we should just remove it and port the function to newer Cocoa calls. If I remove Carbon from linkage, these are the linked symbols I see:

• _SetSystemUIMode (Fixed in Fix non-native fullscreen not hiding menu/dock in secondary screens #1155 )
• _TISCopyCurrentASCIICapableKeyboardInputSource
• _TISCopyCurrentKeyboardInputSource
• _TISCopyInputSourceForLanguage
• _TISGetInputSourceProperty
• _TISSelectInputSource
• _kTISNotifySelectedKeyboardInputSourceChanged
• _kTISPropertyInputSourceID

Go through them and port them.