It happens that the update moved all your previous configuration to the files .conf~previous. If you move /etc/apache2/extra/httpd-userdir.conf~previous to /etc/apache2/extra/httpd-userdir.conf, and /etc/apache2/extra/httpd.conf~previous to /etc/apache2/extra/httpd.conf, it should work like before.
# mv /etc/apache2/extra/httpd-userdir.conf /etc/apache2/extra/httpd-userdir.conf~high-sierra
# mv /etc/apache2/httpd.conf /etc/apache2/httpd.conf~high-sierra
# mv /etc/apache2/extra/httpd-userdir.conf~previous /etc/apache2/extra/httpd-userdir.conf
# mv /etc/apache2/httpd.conf~previous /etc/apache2/httpd.conf
# apachectl restart