Some distros could have configuration files in a directory different than /etc/. For example openSUSE switches to /usr/etc: UsrEtc packaging guideline

greetd has it hardcoded and only it

Would be nice to have options for building greetd that would take as a parameter vendor location and greetd first checks /etc/ and then vendor dir location for the config files.

Vendors dir for pam.d /usr/etc/pam.d instead of /etc/ don't work with greetd.

UsrEtc packaging guideline

Actually greetd checks for the path absolute path for the pam.d.

which means that it will error out. Workaround is to link /etc/pam.d/greetd to /usr/etc/pam.d/greetd .

for more details see this issue