Other new features include enhanced audio return channel, variable refresh rate, quick media switching, quick frame transport, auto low latency mode, and display stream compression (DSC). The 2.1 standard actually allows 8K@120Hz and 10K@120Hz, but only through DSC. A new "Ultra High Speed HDMI" cable with 48 Gbps throughput will also be required.
https://www.notebookcheck.net/Final-HDMI-2-1-specifications-released-with-8K-10K-and-dynamic-HDR-support.267392.0.html