How is the title of "National treasure" attained? Ushio Hinomaru is one of the "National treasures", with the special title of "Onimaru Kinutsuna", along with a bunch of other very strong wrestlers, but at how was the title awarded? 

Because Hinomaru ghosted at some point but still retained the title, How was that possible? How does it work and is it a lifetime kinda award?
I've yet to read this but I think I found the answer. Hinomaru is someone who has the title 'National Treasure' because he


  has the potential to become a yokozuna. (Wiki)


Yokozuna is the highest rank in sumo. 'National Treasure' is more like a moniker or an informal title rather than a formal one that can be obtained. It is not an official rank but a nickname and since only those with a potential to attain the highest rank are given this nickname, then whether or not he ghosted, he still has the potential to become yokozuna, hence he was able to keep the 'title'.