The purpose of UnknownContentHandler and UnknownContent is to allow content handlers to be undeployed from a wiki that still has content in the database that is declared to use the respective model. With the new MCR enabled storage layer, old "model oblivious" access methods no longer work.
In such a case, UnknownContentHandler could be register as the handler for the now unknown content model, to avoid hard failure modes while generating dumps and diffs.
UnknownContent would wrap the serialized blob, and return it unchanged for serialization. Rendering would produce empty output or output containing an error message. Diffing would similarly only produce a message.
UnknownContent would not allow direct editing. Pages containing UnknownContent in the latest revision would become immutable.