There’s no such thing as USB hotspots, that’s a term for WiFi. Also you can still use the NMC protocol if your Android version is recent enough. Just not RNDIS anymore. It’s an insecure Microsoft protocol, though this probably wouldn’t have mattered for a lot of people.

It’s not about improving a connection, but making your phone’s network connection available to other devices. USB tethering creates a network device at /dev/usb... that behaves like an any ordinary network device, allowing you to create a connection using it. Wi-Fi tethering creates a hotspot similar to what your router at home does.

That’s different protocols. This is only about the one being used when you share your phone’s connection via USB (tethering). Neither adb nor fastboot make use of this.

Also, starting with Android 14, there should be an alternative available with NCM.