Option Shift word selection doesn't work in Visual Studio Code on Mac OS Big Sur
The fact that opt+shift+up/down work seems to suggest that this is an keybinding issue.
Under your VSCode keybindings, check to see if the keybinding cursorWordLeftSelect and the corresponding right select ones are active. If they are active, check to see if any other extensions override these shortcuts (by searching for the keybind: opt+shift+left).

Hopefully this fixes the issue.