It was a relatively recent change [1]. Try the latest Gnome OS nightly ISO in a VM -- you'll see that they've (largely) implemented the partition scheme suggested in ParticleOS: root on btrfs, two partitions for /usr backed by dm-verity, new /usr images delivered using "systemd-sysupdate".

[1]https://www.osnews.com/story/139696/gnome-os-is-switching-fr...

Very interesting. Thanks.

I will indeed have a look, ASAP -- but I hope this version is a little more tolerant of non-GNOME/non-RH hypervisors, or I won't get far...