> In hindsight, how would you have used the documentation of the Raspberry Pi architecture to determine whether UASP is supported?

Doesn't really depend on the Pi hardware at all, it depends on the kernel driver (kernel must be new enough) and the USB storage device (does it implement UASP in addition to legacy bulk-only-transport?).

https://www.devever.net/~hl/usbuas has a great explanation of the nitty-gritty details.