When working on IBX-538, the entire eZ\Publish\SPI\Persistence namespace was automatically moved to Ibexa\Contracts\Core\Persistence. This is not desired end-result as those interfaces were never frozen and were never given BC promise. Keeping them in Contracts might lead to difficulties in maintenance because we wouldn't be able to modify internals of legacy storage without breaking a contract.

Challenges: the namespace needs to be moved to Ibexa\Core\Persistence which might lead to name conflicts which need to be resolved.

If we were to design Storage abstraction which is indeed BC-safe for 3rd party (as SPIs), it should be done from scratch.