It's been reverted.
commit 95ab9eff1e24cf996cd4111a93e3a8b29ff0f524
Author: Andrew Jeddeloh
Date: Thu Oct 5 03:58:02 2017 -0700
Revert "networkd: change UseMTU default to true. (#6837)" (#6950)
This reverts commit 22043e4317ecd2bc7834b48a6d364de76bb26d91.
UseMTU is broken on real
I saw in https://github.com/systemd/systemd/pull/6936 that networkd's
UseMTU will be enabled by default in 235. There is an bug
https://github.com/systemd/systemd/issues/6593 that causes networkd to
loop continually setting and unsetting the MTU on most real hardware
if UseMTU is set and the the DH
