Allowing custom code to save/restore metadata about a card would allow people to experiment with schedulers further removed from the standard SM2 algorithm. We could do this by packing an object with arbitrary keys into card.data["meta"], and exposing a symbol like meta that the JS code could read from/write to.

https://forums.ankiweb.net/t/some-problems-in-implementing-a-state-of-the-art-srs-scheduler-on-anki/22705

@RumovZ feel free to tackle this if it interests you