We want to be able to write extra slots, such as the media-info slot for SDC, before we disable support for the old schema. Otherwise, enabling additional slots would be blocked on T183487: MCR schema migration stage 3: stop using legacy fields, which is probably going to take quite a while, since it means updating all legacy code that accesses the old database fields directly.
Implementing this should be as simple as removing a check that makes the RevisionStore reject revisions with extra slots.