man utmp

"POSIX.1 does not specify a utmp structure, but rather one named utmpx, with specifications for the fields ut_type, ut_pid, ut_line, ut_id, ut_user, and ut_tv.  POSIX.1  does  not  specify  the  lengths  of  the ut_line and ut_user fields."

То есть файл есть, но структура не стандартизована

"Linux utmp entries conform neither to v7/BSD nor to System V; they are a mix of the two. v7/BSD  has  fewer  fields;  most  importantly it lacks ut_type, which causes native v7/BSD-like programs to display (for example) dead or login entries.  Further, there is no configuration file which allocates slots to sessions.  BSD does so because it lacks ut_id fields. In Linux (as in System V), the ut_id field of a record will never change once it has been set, which reserves that slot without needing a configuration file.  Clearing ut_id may result in race conditions  lead‐ing  to corrupted utmp entries and potential security holes.  Clearing the abovementioned fields by filling them with null bytes is not required by System V semantics, but makes it possible to run many programs which assume BSD semantics and which do not modify utmp.  Linux uses the BSD conventions for line contents, as documented above. System V has no ut_host or ut_addr_v6 fields."

То есть у линукса свой формат файла ни с чем не совместимый

А что есть в freebsd? Нету там его там utmpx
И не только в ней
https://lists.dragonflybsd.org/pipermail/commits/2019-Septem...

Дальше:
Location
Depending on the system, those files may commonly be found in different places (non-exhaustive list) :

Solaris:
/var/adm/utmp (deprecated), /var/adm/utmpx
/var/adm/wtmp (deprecated), /var/adm/wtmpx

HP-UX:
/etc/utmp (deprecated), /etc/utmpx
/var/adm/wtmp (deprecated), /var/adm/wtmpx
/var/adm/btmp (deprecated), /var/adm/btmpx

FreeBSD 9.0 introduced new files while adding support for utmpx:
/var/run/utx.active (replaces utmp)
/var/log/utx.lastlogin (replaces lastlog)
/var/log/utx.log (replaces wtmp)

То во всех ос свой файл и свой формат.

Вот эти вот люди которые почти 200 комментариев исписали про "прибить к systemd чтобы было несовместимо с другими ос" они точно понимаю о чем пишут?