> there's also no way for a cable to communicate its current limitations

I don't think that's quite right. USB-C cables that support >60W (>3A @ 20V) must have an e-mark, which is a small chip embedded in the cable that identifies it as being able to support specific levels of power, such as 5A @ 20V for 100W or 5A @ 48V for 240W.

Spec-compliant chargers won't supply more than 60W unless the cable has the appropriate e-mark.

Nothing prevents anyone to use the USB-PD protocol over a different cable and plug (and maybe some do but we don't know it) rated for 10A. It's just so much more practical both for users and when designing to just use off the shelf USB-C (which AIUI has its design rated for 5A) as the physical part.

USB-C cables other than basic low speed 3A ones are required to have an embedded e-marker chip that enumerate the capabilities of the cable.