It's almost trivial to do the same without this API: focus/blur events, user input from mouse/keyboard. A permission toggle will add nothing.
> its use in various "ed tech" websites to detect alleged cheating[1] is frightening, to say the least
Because? Monitoring what happens during an exam seems normal and reasonable. There is a limit to what is "reasonable" – monitoring webcam and microphone is not IMHO, but seeing if another tab was opened is fairly reasonable.
It's not during an exam. If you read the link referenced, it notes that Canvas can log activity even when in non-proctored tests, for which there is no indication or expectation of this happening. For proctored tests they just record with your webcam and microphone anyways.
It's still during an exam. The expectation of complete privacy of what happens on an exam webpage while you're taking the exam is an unreasonable expectation.
> its use in various "ed tech" websites to detect alleged cheating[1] is frightening, to say the least
Because? Monitoring what happens during an exam seems normal and reasonable. There is a limit to what is "reasonable" – monitoring webcam and microphone is not IMHO, but seeing if another tab was opened is fairly reasonable.