The utmp file is now obsolete. There should be a warning about that
in the utmp(5) man page.

FYI:

systemd (256.5-2) unstable; urgency=medium
[...]
  [ Luca Boccassi ]
  * Disable utmp support, replaced by wtmpdb. utmp is not y2038-safe, util-
    linux has now turned it off and relies on logind, so disable utmp
    support in logind too, as it is no longer necessary. wtmpdb replaces
    the functionality.