Well what I can say is that since my team migrated everything to LXD/Incus the amount of tickets that are somehow related to the virtualization solution we used dropped really fast. Side note: we started messing around with LXD from Snap (but running under Debian) and moved to Debian 12 version as soon as it was made available.
About the kernel things, my upstream fix comment was about how Canonical / Ubuntu does things. They usually come up with some “clever” ideia to hack something, implement it and then the upstream actually solves the issue after proper evaluation and Ubuntu just takes it and replaces their quick hack. This happens quite frequently and it’s not always a few lines of code, for instance, it happened with the mess that shiftfs was and then the kernel guys come up with a real solution (idmapped) and now you see Canonical is going for it. Proxmox inherits the typical Canonical mess.