Here's the answer ...
UserDir directive is set in /private/etc/apache2/extra/httpd-userdir.conf which is included in httpd.conf .
[01:17 43] grep -i userdir /private/etc/apache2/extra/httpd-userdir.conf
# Required module: mod_userdir
# UserDir: The name of the directory that is appended onto a user's home
UserDir Sites