The unchecked keyword is new in solidity version 0.8.0, so this only applies to that version or higher, which these instances are. This saves 30-40 gas per loop
There is 1 instance of this issue:
https://github.com/code-423n4/2022-05-backd/blob/2a5664d35cde5b036074edef3c1369b984d10010/protocol/contracts/zaps/PoolMigrationZap.sol#L22
