Ubuntu is now has a /usr-merged filesystem layout. See Why are /bin and /sbin now symlinks in Ubuntu 20.04? for more on that. As part of this change, to work around some potential package management problems, these <x>.usr-is-merged directories are created.
M4: Protective diversions for aliasing links
For each of the aliasing symbolic links, we can introduce a diversion
that redirects it to some unimportant location. Since diversions are
not intended to be used with directories, dpkg only applies a
diversion to the exact filename that is being diverted. When adding a
diversion for one of the aliasing symbolic links, files that are
installed below that directory component are unaffected by the
diversion. Any attempt to remove a diverted symbolic link will instead
remove the corresponding unimportant location. In order to avoid a
Pre-Depends loop, the diversions are created by a dependency-less
package (e.g. a new usrmerge-support package). libc6 as the sole
owner of /lib64 needs a Pre-Depends and can do so without
introducing a loop. base-files is a prominent owner of many other
directories that have become symlinks and also needs such a
Pre-Depends. With these in place, P9 is addressed. The diversions
can be removed if the symlinks are installed into some data.tar or
after two stable releases to accommodate external packages and
derivatives.
This is confirmed as an expected state in this Launchpad bug report, and will be cleaned up in 24.10.
