Upgrade 21 is the plumbing before interop, not interop itself. The $OP governance proposal went up Nov 4: an L1 contracts-only change (op-contracts/v9.0.0) giving every OP Stack chain one ETH custody model and one pause identifier, and hardening the L1 withdrawal path. There's no L2 hardfork, so the proposal lists no action for node operators or ordinary users. Sepolia execution is set for Nov 9, the veto window closes Nov 11, mainnet Nov 16 - each pending approval and a clean seven-day bake. Interop's own date isn't in this one.