Debian has decided quiet a while ago they do not want to support different mountpoints other than /usr/ in the booting process anymore. From the origins of Unix you had the option on boot to mount /bin, /lib and /sbin) to different drives. Which made sense because disksizes where limited. Nowadays this is hardly a problem but with a history of decades it would hardly be a problem to keep it this way too.
Anyway this has been decided and starting from Debian Bookworm you do not have a choice. With updating from Bullseye to Bookworm installing usrmerge and usr-is-merged are mandatory. Except it does not work for systems running on NFS and your update will stop and will not continue updating the other packages too. And because usrmerge is mandatory you have to adapt the filesystem on the server to usrmerge manually before you will be able to continue.
Or you do not do that and have a unsupported system. In my use case this network booted system originate from Debian Jessie and will not be continued beyond Bookworm.
So I created two files in /etc/apt/preferences.d to block usrmerge and usr-is-merged from installing/updating.
usrmerge
Package: usrmerge Pin: release * Pin-Priority: -1
usr-is-merged
Package: usr-is-merged Pin: release * Pin-Priority: -1
So far only package init-system-helpers will not update because of depending on usrmerge. But because you did not install usrmerge you probably do not need a new version either.