DNS0 was launched at a time when no other EU public DNS resolver was available. Today, you have DNS4EU that is actively funded and pushed throughout the EU administration (and critical infrastructures), so I believe that the DNS0/NextDNS founders saw that there was very little differenciating factors to their proposal, and decided to shut it down.

Had they "captured" a larger marketshare in the EU while they were ahead, situation might have been different today, but in my opinion it never happened.