It's not commonly found on monitors. DSC is included starting with DisplayPort 1.4, and a good portion of monitors even today still ship with DP 1.2
Nvidia and AMD have not been shipping DSC with HDMI simply because HDMI included DSC starting with HDMI 2.1, which only the current generation of graphic cards and very few monitors support.
Not very commonly found on what? Nvidia & AMD have been shipping DSC for ~2 generations now (so almost anything within the last 4 years has it)