Corrected - I meant leading edge.

Context re:distros mentioned:

GNOME OS: https://os.gnome.org/ KDE Linux: https://kde.org/linux/

GNOME OS uses BuildStream and as a result has no concept of packages at all and no relationship to any distro, KDE Linux is based on Arch. There is no relationship between the two. GNOME OS used to be OSTree based but switched to systemd-sysupdate a while ago.

Sorry, not completely separate, yes, but some of the parts of the standard (e.g. systemd-sysext for layering "packages") and closely related things like systemd-sysupdate do actually replace parts of this (esp. ostree).

bootc and OSTree are both very neat, but the leading edge of immutable Linux distros (GNOME OS, KDE Linux) is currently converging on a different proposal by systemd developers that's standardized by the UAPI Group (https://uapi-group.org/specifications/). It fixes quite a few of the complexities with OSTree (updates are handled by `systemd-sysupdate`/`updatectl` and are just files served via HTTP) and is quite a bit easier to extend with things like an immutable version of the Nvidia drivers or codecs thanks to system extensions handled by `systemd-sysext` (which in turn are just simple squashfs files overlayed over `/usr`) and configuration via `systemd-confext`. `mkosi`, also by systemd, is quickly becoming _the_ way to build custom images too, and is somewhat tied to these new standards.