By default, users cannot set user services to run at boot time. The admin must enable this on an individual basis for each user. If lingering is enabled for a specific user, a user manager is spawned for the user at boot and kept around after logouts. This allows users who are not logged in to run long-running services. Takes one or more user names or numeric UIDs as argument. If no argument is specified, enables/disables lingering for the user of the session of the caller.

loginctl enable-linger $USER
loginctl disable-linger $USER

Or alternatively:

touch /var/lib/systemd/linger/$USER
rm /var/lib/systemd/linger/$USER

You can show a list of lingering users with:

ls /var/lib/systemd/linger

Or alternatively:

 loginctl user-status $USER

To check user lingering status in scripts (programmatically):

loginctl show-user "$USER" --property=Linger | grep -q 'yes'