I think that in terms of copyright, copying the way Youtube deals with it is good enough imo. So, instead of deleting a video, you would “hide” the deck for new users. On Youtube, people that have seen the video already consumed the content. Leaving the deck available for users already using it kinda follows the same argument of consuming the product.
You could also direct all the possible profit to the copyright owners, just like Youtube does with the monetizing.
You would also have to figure out a way to stop people from uploading this same deck a bunch of times. If not, it would break the rule I mentioned above (users using the deck before it disappearing are still capable of using it).
Also, uploading a list of vocab from a book as a deck is different than uploading Wanikani or iKnow decks. So your policy for the latter would have to be more extreme.
Tbh, Kitsun is not there yet to be worried about this stuff though.
I’m very sceptical when it comes to a tiered product. I think the free version must be improved, that’s all. It doesn’t feel just right yet. The sub will come with more features.