If they don't claim their airdropped Graphene, it is their fault and those unclaimed Graphene will be lost. Total supply will decrease consequently.

With small altcoins, it is the best if you have your own wallets with seeds and private keys. You can control your assets and avoid any troubles from delist, scam exchanges or if you want to claim airdrop, just following guides from the team. Relying on exchanges to support and claim airdrops for you are risky and sometimes you won't get it on any exchange.

Even a big exchange like Poloniex rejected to claim and distribute airdrops in 2018. It is one of reasons they fell down from the top but as a user, you are the one gets loss first.

Graphene team can do burn if they need to reduce total supply of Graphene.