Just stumbled upon this on upgrade to Buster on a IPsec gateway, where the dummy interface is as tunnel source IP.
My solution was to let systemd-networkd manage the device by adding /etc/systemd/network/00-dummy.netdev with [NetDev] Name=dummy0 Kind=dummy and then the standard /etc/network/interfaces stanza for dummy0, followed by `systemctl restart systemd-networkd` and `ifup dummy0`. Everything is back without overriding /lib/modprobe.d/systemd.conf I thing the note should suggest the systemd-networkd solution first, as Debian adopted systemd. Thanks Petr
