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.

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).