[Win 10 Home, 64-bit, Build 20H2, fully updated + MS Office +...]
I just updated NVidia GeForce experience to V188.8.131.52 and it has provided/updated the in-game overlay that uses a number of keyboard shortcuts commonly used by other applications (e.g. Alt +F8, Alt+F11 in Excel).
NVidia GeForce experience steals them: I realised this when Alt+F11 etc. no longer worked in Excel and recalled that NVidia was updated just last night.
By going into Settings for the ing-name overlay and hitting spacebar for each selected NVidia keyboard shortcut I was able to set them all (except Alt-Z to open the overlay!) to None and so restored the functionality of all my other apps.
Apart from being absolutely appalling behaviour on NVidia's part - for which they should be loudly rebuked - it raises this important question for future reference in cases where the culprit is not so obvious:
how can one identify what program is stealing/eating/consuming keyboard shortcuts?
(NB the overlay can be disabled in GeForce settings, under General, but something changed without my permission - either it was enabled by the update or keyboard theft was added to an enabled overlay; either way it was NVidia's fault)