Comment 3 for bug 2045668

usr-is-merged should probably be in main, it serves as an indicator that the system is already using the usrmerge layout. From: https://lists.debian.org/debian-ctte/2022/07/msg00019.html

The usrmerge package has been updated to pick up a few fixes from
Ubuntu, and most importantly to provide a new lightweight metapackage,
usr-is-merged, that can only be installed on merged-usr systems, to
provide a way for installers to avoid the additional dependencies of
usrmerge when they set up the filesystem correctly by themselves (eg:
debootstrap), and for users who already completed the transition. It
also gained a flag file that stops the package from updating the
system, clearly marked as making the system unsupported.