ptg discussion: https://etherpad.opendev.org/p/r.6ad6a4b8c378756c66185a7b35f1bcf9#L368

tl;dr current situation is:
ports created by Nova (for example when booting an instance with a network UUID) are deleted by Nova when the instance is deleted
ports created by the user (booting with a port UUID) are kept when the instance is deleted
The same behavior as instance delete also applies to port detach!!! this is not expected by endusers.
There's a request to make this user-configurable, so we were thinking a 'delete_on_detach' field on ports, that can default to the above ^^^, but still be user-settable

the neutron team suggested using the tags api to track this.
https://docs.openstack.org/neutron/latest/contributor/internals/tag.html

ideally we woudl prefer a key vlaue api rather then the current tags api but we need to look at it more closely.

neutron were open to an tags v2 that supported key value pairs if we needed it.

we also discussed the precedence.

once nova starts using this feature to recored the detach on terminate state on the neutron port we should use that value in precedence over the current value we have in the nova db.

we won't remove the value in the nova db for legacy reasons since the neutron api extension is optional and neutron does not have a way to make extensions required.