After testing on Arch (no netplan)
[IPv6AcceptRA] UseAutonomousPrefix=no UseOnLinkPrefix=no
in /etc/systemd/network/eth1.network will get applied. Anywhere else it won't.
So it's up to netplan to add this config option so it can be generated into /run/systemd/network files.
After testing on Arch (no netplan)
[IPv6AcceptRA] efix=no
UseAutonomousPr
UseOnLinkPrefix=no
in /etc/systemd/ network/ eth1.network will get applied.
Anywhere else it won't.
So it's up to netplan to add this config option so it can be generated into /run/systemd/ network files.