As for VMs, we could add limited routed NIC support to VMs, and just perform the host-side configuration and make it clear in the docs that the user needs to configure the VM side network settings themselves (either manually or via something like cloud-init).
As it stands, most users of routed NICs have to do this with containers anyway, as the default network config in our images often causes the addresses and routes added to the NIC to be wiped out when the container starts anyway.
Hence this guide to use cloud-init: How to get LXD containers get IP from the LAN with routed network
So the experience wouldn’t be so different anyway.