r350184 changed login(1) to read /var/run/motd. As been noted in https://lists.freebsd.org/pipermail/svn-src-head/2019-July/126776.html there is a need to support displaying an immutable motd with specific exact text at login. To keep motd immutable and prevent it being prepended with uname output, update_motd needs to be set to "NO" in rc.conf to not generate /var/run/motd. An immutable motd file can then can be created in /var/run but if it is a tmpfs, the file will not survive a reboot. https://reviews.freebsd.org/D26654 Phabricator review to change login(1) back to reading /etc/motd. This allows using /etc/motd as a regular file when update_motd="NO". update_motd="YES" causes /etc/motd to be created as a symbolic link to /var/run/motd so login(1) will still read it.
The problem also affects update_motd="NO" when not using tmpfs for /var/run. cleanvar_enable="YES" by default and cleanvar scripts removes files under /var/run so if motd is created there manually it will be deleted after reboot.
login(1) must honor the variable from /etc/login.conf: welcome=FOLDER_TO/motd /etc/rc.d/motd needs to update motd with kernel info if update_motd="YES" so /etc/rc.d/motd needs to read the /etc/login.conf and find the motd file and update it. If the filesystem where the motd file is read-only, display a message to inform the user/sysadmin.