Ubuntu 20.04 on Nano?

We do not plan for kernel and Ubuntu upgrade for JetPack 4 codeline and hence Jetson Nano and Jetson TX2 which will remain with JetPack 4 codeline, will not get the kernel and ubuntu upgrades. However, we will continue supporting Jetson Nano and Jetson TX2 on JetPack 4 codeline by making periodic releases containing critical bug fixes and security updates.

See also: Jetson Software Roadmap for 2H-2021 and 2022 - Jetson & Embedded Systems / Jetson Nano - NVIDIA Developer Forums