What problem does this address?

The Interactivity Router store contains details that are internal and are not part of the public API, like for example:

What is your proposed solution?

Those internal properties should be taken out of the public store and moved to a private store instead, so they are not used as public APIs. We can keep them with some deprecation warnings during one or two major versions of WordPress, though.

The namespace of this store could be core/router/private and it should be a locked store.