Comment 12 for bug 1771886

A more elegant way to implement this workaround would probably be to put that configuration snippet into a drop-in override (as described here https://www.freedesktop.org/software/systemd/man/systemd.network.html), e.g.:

$ cat /etc/systemd/network/10-netplan-br0.network.d/override.conf
[Network]
IPv6SendRA=yes
DHCPv6PrefixDelegation=yes
IPv6PrivacyExtensions=yes