That's actually untrue because for USB2 vs USB3 my computers already showed notifications that the USB2 cable is slower than necessary. The computer KNOWS the other device, it has a driver for it, and the driver can say "this device is capable of more".

Sometimes yes, other times it might be possible in another way, but a lot of drivers are generic and don't have an exhaustive list of all devices in existence. Devices only need to be identified as a certain (generic) type.

It's still certainly possible for a device to report it's capabilities and plenty of devices seem to correctly do it already.

It would know, the wSpeedsSupported field on the device tells the host which speeds the device is capable of.

but it could at least report the capacity of the cable. that's still useful when you get a badly advertised cable that you thought could do more, or worse, got a fake cable.