Followup, just re-checked a Debian Stretch installation. There it's all OK, the offending lines are missing from the prerm script. This then seems to be a regression which was triggered by the newer compat level 12 and the different behavior of dh_installsystemd under that level.
- Bug#945877: /var/lib/dpkg/info/ifupdown.prerm: prerm scri... Thomas Lamprecht
- Bug#945877: ifupdown.prerm: script stops networking.... Thomas Lamprecht