Comment 9 for bug 1671951

I've just seen that upstream systemd v239 claims to support IPV6MtuBytes

https://lwn.net/Articles/758128/

* networkd's .network files now support a new IPv6MTUBytes= option for
setting the MTU used by IPv6 explicitly as well as a new MTUBytes=
option in the [Route] section to configure the MTU to use for
specific routes. It also gained support for configuration of the DHCP
"UserClass" option through the new UserClass= setting. It gained
three new options in the new [CAN] section for configuring CAN
networks. The MULTICAST and ALLMULTI interface flags may now be
controlled explicitly with the new Multicast= and AllMulticast=
settings.