I've had a similar experience, my Xavier NX stopped working after the last update and now it's just collecting dust. To be honest, I've found the Nvidia SBC to be more of a hassle than it's worth.
My Jetson TX2 developer kit didn't stop working, but it's on a very out of date Linux distribution.
Maybe if Nvidia makes it to four trillion in market cap they'll have enough spare change to keep these older boards properly supported, or at least upstream all the needed support.
Back in 2018 I've been involved in a product development based on TX2. I had to untangle the entire nasty mess of Bash and Python spaghetti that is JetPack SDK to get everything sensibly integrated into our custom firmware build system and workflow (no, copying your application files over prebaked rootfs on a running board is absolutely NOT how it's normally done). You basically need a few deb packages with nvidia libs for your userspace, and swipe a few binaries from Jetpack that have to be run with like 20 undocumented arguments in right order to do the rest (image assembly, flashing, signing, secure boot stuff, etc), the rest of the system could be anything. Right when I was finished, a 3rd party Yocto layer implementing essentially the same stuff that I came up with, and the world could finally forget about horrors of JetPack for good. I also heard that it has somewhat improved later on, but I have not touch any NVidia SoCs since (due to both trauma and moving to a different field).
Edit: It's been a while since I did this, but I had to manually build the kernel, overwrite a dtb file maybe (and Linux_for_Tegra/bootloader/l4t_initrd.img) and run something like this (for xavier)
If you spent enough time and energy on it.. I'm fairly confident you could get the newest Ubuntu running. You'd have to build your own kernel, manually generate the initramfs, figure out how to and then flash it. You'd probably run into stupid little problems like the partition table the flash script makes doesn't allocate enough space for the kernel you've built.. I'm sure there would be hiccups, at the very least, but everything's out there to do it.
Wait, my AGX is still working, but I have kept it offline and away from updates. Do the updates kill it? Or is it a case of not supporting newer pytorch or something else you need?
