Grayscale shortcut on Big Sur

Given all the Accessibility keyboard shortcuts, you would think toggling greyscale should be one of them, but it's not:

Mac accessibility shortcuts - Apple Support

The best I can suggest is to ask Siri: "turn on greyscale" / "turn off greyscale". That works.

Yes, there is a Siri keyboard shortcut:

The default shortcut to ask Siri is to press and hold the Command key and the Space bar at the same time.

To create your own shortcut, click the Keyboard Shortcut pop-up menu, choose Customize, then press the keys you want to use. For example, you could press Option-Z.

Change Siri preferences on Mac - Apple Support