I just tried to do this and Apple has obfuscated that there’s even a 16:10 option. You have to click deep into the Displays customization presets menu. They don’t even label the presets with a display ratio and have them labeled with some obscure Apple design lingo (Apple XDR (P3-1600 nits)).
To change it you have to first display the hidden list by enabling a “display resolutions list” toggle.
That is not something a “I love Apple because it just works” person can figure out.