Trying to switch to systemd-resolved for various reasons, I find that
wg-quick no longer works.
The problem is that it treats the presence of a conffile
(/etc/resolvconf/interface-order) as the indicator of whether
/usr/sbin/resolvconf is resolvconf (instead of systemd-resolved's drop-in),
but of course conf files can be left behind.
Ubuntu fixed this under LP#2119260 with d/p/0005-wg-quick-linux-deal-with-resolvconf-migration-more-g.patch
which extends this to check if resolvconf is a symlink. This seems like a
good patch for Debian to bring in too.
Workaround: for now I can just purge resolvconf to fix the problem