Brain fart on my end:
/etc/login.defs: MAIL_CHECK_ENAB no
Not profile.d. Sorry! Yes, that is system wide.
In addition to the previous directive, I have the following in my ~/.bashrc:
For many years I have commented out the whole biff snippet in /etc/profile. I don't bother redirecting anything to /dev/null.
I have all system emails and root emails forwarded to my normal user account. I do this through /etc/mail/aliases:
# Who gets root's mail.
With $USER and $HOSTNAME replaced with the actual names. After editing and saving /etc/mail/aliases, run the newaliases command and restart sendmail.