Is this the only place where we have foreign key relationships between the identity backend and something else? If so, maybe this isn't as big of a deal since it's all required for a federated deployment. If you're deploying keystone with federation, you'll need to have a sql backend of some kind to store information required for federation. With the development of shadow users, it is also expected that a separate identity backend (e.g. sql) is available to persist the local (federated) user references.
Is this the only place where we have foreign key relationships between the identity backend and something else? If so, maybe this isn't as big of a deal since it's all required for a federated deployment. If you're deploying keystone with federation, you'll need to have a sql backend of some kind to store information required for federation. With the development of shadow users, it is also expected that a separate identity backend (e.g. sql) is available to persist the local (federated) user references.