The Redshift user service apparently tries to run for any logged-in
user, regardless of whether that user has an X11 environment and a
useful DISPLAY environment variable, which it seems to depend on.

I suppose this is fine if I only log in via an X11 display manager,
but when I log in at the kernel console as a user not already logged
in with an X environment running (e.g., to administer the system as
the superuser, or in case X wasn't working), this becomes problematic,
as Redshift tries to start, fails, and keeps trying, spewing error
messages into the journal seemingly until the system is shut down.

I would suggest that the Redshift service be prevented from starting
unless and until it would have access to an X11 environment and a
useful DISPLAY variable, which I haven't presently sufficient
knowledge of systemd to effect.