To "Guessed", and everyone else who has contributed so far, thank you for helping by identifying:

Application.CommandBars.ExecuteMso "ViewFocusModeView"

and the route to finding it.

To get to its neighbour in the "View > Immersive" group (the Immersive Reader view) using VBA, it's the rather more cryptically named:

Application.CommandBars.ExecuteMso "ToggleLearningTools"

which works in my Office 2021, and might in some earlier versions, too.