No, you can install Incus on any linux distro. I use it on a Debian server.

It’s IncuOS that is immutable.

But proxmox is actually just a set of packages on top of debian, and proxmox is not immutable either. You aren’s supposed to tinker with the host, but if you are doing things like jailbreaking consuner gpu’s to unlock the server gpu only features, then it becomes nice to have that option.