The headline is a little misleading; umtpx has been in DragonFly forever, but now utmp is really retired and programs adjusted to match.  The change is not that user-affecting and utmp data is still accessible; this is part of the ABI change alluded to over the past week.

If you are not familiar with utmp(5) and utmpx(5), they are databases in /var that track user logins and system restarts.  utmpx is of course better cause it has an X.