Desktop profile switching USE default to elogind
Modern desktop environments make use of PAM session tracking for users, login sessions and seats.  The most user-visible part of that is device and file permissions management and reboot/shutdown handling without superuser rights.
Users with systemd can stop reading here and continue with their daily routine.
ConsoleKit2 is unmaintained upstream for more than two years . There are many longstanding bugs and papercuts with consumers that aren't being fixed, not least because these code paths receive very little testing.
Enter the elogind project , which is a standalone logind implementation based on systemd code, currently maintained by a fellow Gentoo user. We have had sys-auth/elogind available in Gentoo since the beginning of 2017, and meanwhile it has gained support  in KDE Plasma, Gnome , Cinnamon, MATE and Xfce, as well as most other former consolekit consumers.
Consequently, the desktop profile is switching away from consolekit to elogind. Users of sys-auth/consolekit who selected a different profile should consider doing the same. A guide is available . Migration is easy, but any existing consolekit session will be broken, and elogind will only begin to work on relogin.
Rely either on the profile, or set USE="elogind -consolekit" in make.conf yourself. Make sure there is no consolekit debris in /etc/portage/package.use:
# grep -R consolekit /etc/portage/package.use
Rebuild all affected consumers and remove sys-auth/consolekit:
# emerge --ask --changed-use --deep @world # emerge --depclean consolekit
Optional, but recommended in case of trouble such as missing reboot/shutdown capabilities in the display manager:
# rc-update add elogind boot
For users of ~/.xinitrc instead of one of the supported DMs, do not forget to update accordingly (ck-launch-session is gone without replacement).
PS: Subsequently, this will lead to the last-riting of sys-power/pm-utils  which is dead even longer than the original ConsoleKit(1) project. KDE Plasma users sticking with sys-auth/consolekit are then going to lose suspend from GUI without superuser rights.