I think that Apple, perhaps naively, expects display manufacturers to adhere to spec when in reality they often don’t.
Either way macOS has no trouble with my 27” 2560x1440 Asus and Alienware monitors. Both connect with 10bit RGB no problem, at least over USB-C and DisplayPort (haven’t tried HDMI).
macOS really wants to do different things for TVs vs monitors, so if it decides your monitor is a TV for whatever reason, it’ll probably prefer YCbCr and also not offer any HiDPI modes except exactly 2x
2560x1440 is a strong indicator of a monitor, but 4k over HDMI tends to get detected as a TV
That makes some amount of sense, I avoid using HDMI if at all possible. Even on other platforms I’ve generally had less trouble out of DisplayPort and USB-C DP alt mode. I wish it became standard for TVs to have at least a single DisplayPort, it’d make hooking computers up more pleasant.
Either way macOS has no trouble with my 27” 2560x1440 Asus and Alienware monitors. Both connect with 10bit RGB no problem, at least over USB-C and DisplayPort (haven’t tried HDMI).