Issue [12.5.30 -> 17.5.3] After upgrading, some (all?) reconfigured sites have wrong DirectoryIndex config

[burnley]

CentOS 7, 12.5.30->17.5.3 in-place upgrade. It appears that every site reconfigured after the upgrade gets the following DirectoryIndex value in httpd.conf, for both http and https:
DirectoryIndex "index.html" "index.cgi" "index.pl" "index.php" "index.xhtml" "index.htm" "index.shtml"
Question: Where do "index.cgi" and "index.pl" values come from? A recursive grep for these 2 in the server-wide Apache configuration return nothing, the only entries returned by "grep -R 'index.cgi' /etc/httpd/conf*" are the symlinks from /etc/httpd/conf/plesk.conf.d/vhosts/ pointing to the sites reconfigured by Plesk. Additional locations searched with no results returned:
1. /usr/local/psa/admin/conf/templates/
2. psa database.
Where does this setting come from, why all of a sudden and at what point exactly? I know there already are articles documenting how to reconfigure the sites as well as the server defaults, like:
Plesk uses directoryIndex of main Apache configuration file
How to change the default index for all websites
But I want to know why this has been happening since the upgrade from 12.5.30 to 17.5.3