Now the RaspberryPI 4 is announced and is going to use Debian Buster as image OS what are the problems that are going to spoil the usage of Pi-hole on that RaspberryPI version?
The feature request is to adapt Pi-hole so that is compatible with Debian Buster.
Update: the solution by bene987 made it possible to use Lighttpd on Buster with the Pi-hole default config for Lighttpd. After that the default (Master) of Pi-hole should have the adaptation by the Developers.
Update 2: Pihole is now patched for this and the version number has increased increased to 4.31.