My workaround was to restore pre-notch behaviour by picking a resolution from the "show all resolutions" list that is conveniently+ exactly screen res height minus notch height.

I theoretically "lose" that much height but gain a) zero notch b) non-rounded top corners and c) a traditionally heighted menubar instead of the giant one that is so big only to cater for the notch.

+ I thought this was thanks to BetterDisplay but it turns out no third party tool is needed and it's all first party probably because someone at Apple is as annoyed by the notch as I was and so that's their solution.

I think it is because they want to send to apps resolution list that includes or excludes the notch area to choose from for full screen modes (eg in games). Selecting "show all resolutions" basically shows this list.