According to https://doc.powerdns.com/ (landing page) the 3.x branch of PDNS is unsupported.
This is what we have right now in cloudservices1003.wikimedia.org:
aborrero@cloudservices1003:~ $ dpkg -l pdns-* Desired=Unknown/Install/Remove/Purge/Hold | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad) ||/ Name Version Architecture Description +++-=============================-===================-===================-=============================================================== un pdns-backend <none> <none> (no description available) ii pdns-backend-mysql 3.4.1-4+deb8u8 amd64 generic MySQL backend for PowerDNS ii pdns-recursor 4.0.4-1+deb9u3~bpo8 amd64 PowerDNS Recursor ii pdns-server 3.4.1-4+deb8u8 amd64 extremely powerful and versatile nameserver
Luckily PDNS 4.0.3-1~bpo8+1 is present in Debian's jessie-backports and apparently in good shape (https://tracker.debian.org/pkg/pdns).
Shall we try migrating from 3.x to 4.x?