Do not allow the challenger to delete finalized outputs

packages/contracts-bedrock/.gas-snapshot

@@ -146,27 +146,28 @@ L2ERC721Bridge_Test:test_finalizeBridgeERC721_notViaLocalMessenger_reverts() (ga
 L2ERC721Bridge_Test:test_finalizeBridgeERC721_selfToken_reverts() (gas: 17659)
 L2ERC721Bridge_Test:test_finalizeBridgeERC721_succeeds() (gas: 168970)
 L2OutputOracleTest:test_computeL2Timestamp_succeeds() (gas: 37206)
-L2OutputOracleTest:test_constructor_badTimestamp_reverts() (gas: 70812)
-L2OutputOracleTest:test_constructor_l2BlockTimeZero_reverts() (gas: 45808)
-L2OutputOracleTest:test_constructor_succeeds() (gas: 33695)
-L2OutputOracleTest:test_deleteL2Outputs_afterLatest_reverts() (gas: 211900)
-L2OutputOracleTest:test_deleteL2Outputs_ifNotChallenger_reverts() (gas: 18894)
+L2OutputOracleTest:test_constructor_badTimestamp_reverts() (gas: 70838)
+L2OutputOracleTest:test_constructor_l2BlockTimeZero_reverts() (gas: 45856)
+L2OutputOracleTest:test_constructor_succeeds() (gas: 33760)
+L2OutputOracleTest:test_deleteL2Outputs_afterLatest_reverts() (gas: 212279)
+L2OutputOracleTest:test_deleteL2Outputs_finalized_reverts() (gas: 107742)
+L2OutputOracleTest:test_deleteL2Outputs_ifNotChallenger_reverts() (gas: 18895)
 L2OutputOracleTest:test_deleteL2Outputs_nonExistent_reverts() (gas: 107337)
-L2OutputOracleTest:test_deleteOutputs_multipleOutputs_succeeds() (gas: 302143)
-L2OutputOracleTest:test_deleteOutputs_singleOutput_succeeds() (gas: 180722)
+L2OutputOracleTest:test_deleteOutputs_multipleOutputs_succeeds() (gas: 302544)
+L2OutputOracleTest:test_deleteOutputs_singleOutput_succeeds() (gas: 181101)
 L2OutputOracleTest:test_getL2OutputIndexAfter_multipleOutputsExist_succeeds() (gas: 267204)
-L2OutputOracleTest:test_getL2OutputIndexAfter_noOutputsExis_reverts() (gas: 17936)
-L2OutputOracleTest:test_getL2OutputIndexAfter_previousBlock_succeeds() (gas: 96064)
-L2OutputOracleTest:test_getL2OutputIndexAfter_sameBlock_succeeds() (gas: 96038)
-L2OutputOracleTest:test_getL2Output_succeeds() (gas: 101721)
+L2OutputOracleTest:test_getL2OutputIndexAfter_noOutputsExis_reverts() (gas: 17959)
+L2OutputOracleTest:test_getL2OutputIndexAfter_previousBlock_succeeds() (gas: 96087)
+L2OutputOracleTest:test_getL2OutputIndexAfter_sameBlock_succeeds() (gas: 96016)
+L2OutputOracleTest:test_getL2Output_succeeds() (gas: 101699)
 L2OutputOracleTest:test_latestBlockNumber_succeeds() (gas: 96983)
 L2OutputOracleTest:test_nextBlockNumber_succeeds() (gas: 17512)
-L2OutputOracleTest:test_proposeL2Output_emptyOutput_reverts() (gas: 26688)
-L2OutputOracleTest:test_proposeL2Output_futureTimetamp_reverts() (gas: 28646)
+L2OutputOracleTest:test_proposeL2Output_emptyOutput_reverts() (gas: 26711)
+L2OutputOracleTest:test_proposeL2Output_futureTimetamp_reverts() (gas: 28690)
 L2OutputOracleTest:test_proposeL2Output_notProposer_reverts() (gas: 25804)
-L2OutputOracleTest:test_proposeL2Output_proposeAnotherOutput_succeeds() (gas: 101071)
+L2OutputOracleTest:test_proposeL2Output_proposeAnotherOutput_succeeds() (gas: 101049)
 L2OutputOracleTest:test_proposeL2Output_unexpectedBlockNumber_reverts() (gas: 28402)
-L2OutputOracleTest:test_proposeL2Output_unmatchedBlockhash_reverts() (gas: 29424)
+L2OutputOracleTest:test_proposeL2Output_unmatchedBlockhash_reverts() (gas: 29402)
 L2OutputOracleTest:test_proposeL2Output_wrongFork_reverts() (gas: 29005)
 L2OutputOracleTest:test_proposeWithBlockhashAndHeight_succeeds() (gas: 95274)
 L2OutputOracleUpgradeable_Test:test_initValuesOnProxy_succeeds() (gas: 26141)

packages/contracts-bedrock/contracts/L1/L2OutputOracle.sol

@@ -143,6 +143,12 @@ contract L2OutputOracle is Initializable, Semver {
             "L2OutputOracle: cannot delete outputs after the latest output index"
         );
 
+        // Do not allow deleting any outputs that have already been finalized.
+        require(
+            block.timestamp - l2Outputs[_l2OutputIndex].timestamp < 7 days,
+            "L2OutputOracle: cannot delete outputs that have already been finalized"
+        );
+
         uint256 prevNextL2OutputIndex = nextOutputIndex();
 
         // Use assembly to delete the array elements because Solidity doesn't allow it.

packages/contracts-bedrock/contracts/test/L2OutputOracle.t.sol

@@ -411,6 +411,20 @@ contract L2OutputOracleTest is L2OutputOracle_Initializer {
         vm.expectRevert("L2OutputOracle: cannot delete outputs after the latest output index");
         oracle.deleteL2Outputs(latestOutputIndex - 2);
     }
+
+    function test_deleteL2Outputs_finalized_reverts() external {
+        test_proposeL2Output_proposeAnotherOutput_succeeds();
+
+        // Warp past the finalization period + 1 second
+        vm.warp(block.timestamp + 7 days + 1);
+
+        uint256 latestOutputIndex = oracle.latestOutputIndex();
+
+        // Try to delete a finalized output
+        vm.prank(owner);
+        vm.expectRevert("L2OutputOracle: cannot delete outputs that have already been finalized");
+        oracle.deleteL2Outputs(latestOutputIndex);
+    }
 }
 
 contract L2OutputOracleUpgradeable_Test is L2OutputOracle_Initializer {