Comment 4 for bug 1671634

Yes, the initial MTU blueprint actually meant to make it writable: https://specs.openstack.org/openstack/neutron-specs/specs/kilo/mtu-selection-and-advertisement.html (The spec was written by Ian Wells back in the days.) Quoting: "the tenant can request a specific MTU on a network - this is so that, in the case that the tenant has VMs with preconfigured MTUs (for instance, ones that do not respect any of the available forms of MTU advertisement, they can ask for a specifc - typically 1500 - MTU for networks that have these VMs on)."

It just never got implemented.