For example, I have a Fiio bluetooth adapter which will not charge with a high rate charger, only a USB-A 5v charger via a USB-A -> USB-C adapter. Is it my MacBook and iPad charger’s fault? The cable’s fault? Fiio’s USB-C chipset fault? Beats me - I have no way to know or find out, since I don’t have the tools to sniff out who is not handling the charging protocol properly (and yes, this could be the cable, since there is silicon attached to the USB-C plugs on both sides of the cable which may be involved in the negotiation as well).

This isn’t a huge problem for me now, since I have very few USB-C cables, and none which are not effectively always plugged into a charger. However, in a decade I’ll have to figure this out with a box of cables with wildly differing capabilities.

Knowing the data rate of a cable is an utter mess. But for charging, every cable can do 60 watts. For most purposes the difference between 60 and 100 watts is pretty minor.

It's possible they exist, but I have yet to have personal experience of an actual Thunderbolt cable that didn't have a thunderbolt mark on the connector.