So I recently opened a support ticket about this card, because I believe the game should be able to see if players have fully unlocked this stuff even if they no longer own it, and was hoping they’d be able to assist in some way, this was the response I got.
This is Game Master Strybur. Customer support doesn’t have a means to assist with such issues or facilitate the request, but I can cover how it should work to get you pointed in the right direction.
To get the Diamond Loatheb you need to have all 30 cards from Naxxramas in your current collection and in many cases, some players may have disenchanted Maexxna or something of that nature. It’s all good though, because it should still be claimable from the Achievements tab in the game, so that would be how to check your progress and go from there.
Now if you’re not seeing it due to the achievement system being added in later or anything like that, then you would need to reach out to the developers through the Hearthstone Bug Forum to submit a bug report and then wait it out while they investigate and look into the matter. They will post any updates or workarounds as they go and thus far many of the cases ended up clearing up on their own over time. It’s just if it doesn’t, then the team will have to look into it to see what other options they might have or if they are able to create any other resolution or workaround.
This leads me to believe that there is supposed to be a system in place, now is this something that’s just not working properly? Or is still being worked on? Because I’m sure myself and many other long time players would appreciate this as otherwise we’re effectively punished for achievements being added so late into this games lifespan if we didn’t hang onto everything.