L4T 28.x and Ubuntu 16.04 future support

I see that Standard Support for Ubuntu 16.04 is due to end in April 2021. As the L4T 28.x series is based on 16.04, what are the plans for future releases of this series?

I’ve taken a good look around the nvidia website but have been unable to find this information. Apologies if I’ve missed it and if this question has been asked before.

I don’t know what those plans are, but you can find the list of various L4T releases here (you might need to go there, log in, and then go there a second time):
https://developer.nvidia.com/linux-tegra

There are many releases in the L4T R32.x which have been out for some time. These are all Ubuntu 18.04. Generally speaking using 18.04 would be the path of continued support.