The tracking generated warnings when the line number of an existing tty in /etc/ttys changed, which would corrupt utmp (as it was indexed by the line number). With utmpx, the line number no longer matters, so the tracking is no longer needed.
The tracking generated warnings when the line number of an existing tty in /etc/ttys changed, which would corrupt utmp (as it was indexed by the line number). With utmpx, the line number no longer matters, so the tracking is no longer needed.