Debian 12 had aimed to have a merged “/usr” file-system layout similar to other Linux distributions, but The Debian Technical Committee earlier this year decided to impose a merged-/usr file movement moratorium. But now with Debian 12 having been out for a few months, that moratorium has been repealed.
In hoping to have the merged /usr layout ready in time for Debian 13 “Trixie”, yesterday that moratorium was repealed. Debian’s UsrMerge Wiki page tracks the effort for those wishing to learn more about this modernization of the directory structure.
Debian’s merged /usr transition will hopefully be all wrapped up in time for the Debian 13 release in about two years time.
The argument was that if you put all your static resources in /usr, you can mount it RO (for integrity, or to use a ROM on something embeddedish) or from a shared volume (it’s not uncommon to NFS mount a common /usr for a pool of managed similar machines).
…that said, many of the same people who made that argument are also the ones that went with making it so systemd won’t boot without /usr populated anymore, so that feature is now less useful because it has to be something your initramfs/initcpio/whatever preboot environment mounts rather than mounted by the normal fstab/mount behavior, and the initcpio/initramfs/dracut schemes for doing that all (1) require a redundant set of tools and network configs in the preboot (2) are different and (3) are brittle in annoying ways.
It still works OK if you’re using a management tool like Warewulf to manage configs and generate all the relevant filesystems and such, but it’s a lot more fucking around than a line in fstab to mount usr once the real system is up like the old days.