[Apple] Sudden greyscale shift on Macbook Air

Best guess is Display Contrast.

That will affect the on-screen image, but will not translate to a screenshot. Screenshots are taken before that filter is reached. The on-screen image will be affected similarly to your Photoshopped version. [It will actually compress the blacks a bit too, so not exactly the same as your image.] Almost like putting a really agressive curve on it -

System Prefs > Accessibility > Display. [from Mojave, things may have moved a bit in Big Sur]
Set Display Contrast hard left for 'normal'. All other checkboxes in there should be off, except shake mouse.

There is a key command for it, though I would expect that by default this would be inactive…
System Prefs > Keyboard > Shortcuts > Accessibility