It seems that currently it's hard to backport any database schema fix to Neutron [1] which uses alembic to manage db schema version. Nova has the same issue before

and a workaround is to put some placeholder files before each release. 

So first do we allow db schema fixes to be backport to stable for Neutron ? 

If we do, then how about put some placeholder files similar to Nova at the end of each release cycle? or we have some better solution for alembic.

 From the stable maintainer side, we have a policy for stable backport  https://wiki.openstack.org/wiki/StableBranch  

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