Some things don't work unless you use the right kind of USB-C cable.

EG your GPU and monitor both have a USB-C port. Plug them together with the right USB cable and you'll get images displayed. Plug them together with the wrong USB cable and you won't.

USB 3 didn't have this issue - every cable worked with every port.

Yeah, I also don't think they've broken backwards compat ever. Super high end charger from 2024 can charge old equipment from 2014 just fine with regular 5V.

What was broken was the promise of a "single cable to rule them all", partly due to manufacturers ignoring the requirements of USB-C (missing resistors or PD chips to negotiate voltages, requiring workarounds with A-to-C adapters), and a myriad of optional stuff, that might be supported or not, without a clear way to indicate it.

I don’t know if it’s the spec or just a plethora of vendors that ignores it, but I have many things with a USB-C port that requires USB-A as source. USB-C to A to C works, yay dongles, but not just C to C. So maybe it’s not really breaking backwards compatibility, just a weird mix of a port and the communication being separate standards.

Yeah - it's a mess. Some devices only charge with a charger that supports PD. Some other devices need a charger WITHOUT PD support.