[znpy]

From https://uapi-group.org/ :

> Contributing members include people from Ubuntu Core, Debian, GNOME OS, Fedora CoreOS, Endless OS, Arch Linux, SUSE, Flatcar, systemd, image-builder/osbuild, mkosi, tpm2-software, System Transparency, buildstream, BTRFS, bootc, composefs, (rpm-)ostree, Microsoft, Amazon, and Meta.

Note systemd, (rpm-)ostree and bootc.

My understanding is that uapi is another initiative but not completely separated from bootc and ostree. Maybe complementary.

[pojntfx]

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).