The new proposal has already been created. See: https://classic-agora.terra.money/t/proposal-burn-the-remaining-ust-in-the-community-pool-cross-chain-liquidity-incentive-ust-2
Considering that Luna Classic will continue to exist after the fork, then we will be able to burn Luna Classic after the fork.
And yes, creating a new chain doesn’t mean Luna Classic cannot be burned.