I do have it in Pixel 8 after enabling in developer options. It's a bit buggy and low resolution, but does the job when e.g. I want to connect some video I'm already watching on mobile to the external display via USB-C. (You can connect a mouse via Bluetooth to the phone, or via a USB dongle plugged into your monitor, to control it.)

An interesting thing is that you can run apps X and Y on desktop screen while also run app X on mobile screen independently.