MDL-48969 the quiz_results block was converted to the stub, solely to support backup/restore. But it can be un-hidden and seen as a faily useless block.
Eloy offered two alternative approaches for the backup and restore:
2) We invent some fallback for this so, when a block does not exist in target site, all available blocks are asked if they support getting "ownership" of any other block. This requires adding such a new facility to restore, but long term it's the best approach I can imagine. Specially when the old and the new block are 99% the same. Basically the new one will implement the ability of restoring the information from both.
3) There is a 3rd way, called coverters, that is pretty much what we use to restore 1.9 backups or IMS-CC packages. Basically it's about being able to generate a 2.x formatted backup from different sources. But surely this is not the solution for this tiny change and should remain available only for real conversions.
Ideally we should get rid of it completely to prevent any confusion.