Note the disclaimer on top of that file, which actually is a link to:
/etc/lighttpd/conf-available/15-pihole-admin.conf
# Pi-hole: A black hole for Internet advertisements
# (c) 2017 Pi-hole, LLC (https://pi-hole.net)
(...)
###############################################################################
# FILE AUTOMATICALLY OVERWRITTEN BY PI-HOLE INSTALL/UPDATE PROCEDURE. #
# ANY CHANGES MADE TO THIS FILE AFTER INSTALL WILL BE LOST ON THE NEXT UPDATE #
###############################################################################
As mentioned in Pi-hole FTL v5.20.1, Web v5.18.1 and Core v5.15 released, custom configurations should go to /etc/lighttpd/conf-available/whatevername.conf, from where it should be linked to /etc/lighttpd/conf-enabled/whatevername.conf.