On a pretty fresh Guix System install, I see some regular sshd error messages on tty1 (which I guess means they’re written to /dev/console). Also, setting up a regular shepherd service via make-forkexec-constructor for a program that logs to stderr (postgrest which I’m in the process of packaging), all output goes to tty1.
Compare the discussion at https://lists.gnu.org/archive/html/guix-devel/2019-06/msg00186.html