iproute2, the very basic network package, all network capabilities are based on. That is very bad and lead already to many broken systems. I'm gonna make this a topic now on the Pi-hole repository, this is long overdue.
Either packages must only be marked as "auto", which allows them to be autoremoved afterwards, IF not other package requires them. Due to installed "ifupdown", this would not happen with iproute2 then. I guess in your case "rc ifupdown" was in the "dpkg -l" output, as the package got removed (as dependant of iproute2) but the config file remained (ii = installed, rc = only config files still present).
Also Pi-hole aims to make the install and grants great free user support, which addresses especially inexperienced users. But giving them an easy choice to break the system on regular uninstall works completely against that aim .