Adding a new untagged vlan to deployed machine

How should I go about adding a new untagged vlan to a machine that is already deployed, since it seems like no new interfaces can be added after deployment?

First approach I thought of was adding a bridge that is attached to a physical interface or a bond but that is not possible due to the reason that deployed machines can’t get any new interfaces added.
The second approach that would actually work would be making a large number of bridges ahead of deployment and then changing vlans when required but this seems wrong.

So my final question is why is it not possible to add a new interface after deployment, if I can do it just by logging into the KVM and putting the new interface into netplan?

Hi, this feature is being discussed inside the team at the moment.

In the meantime, any other thoughts you have or info about your use case would be valuable.