Discussion topic for https://github.com/ethereum/EIPs/pull/2488
If we can’t find any real usage of the CALLCODE opcode, I would rather just have that opcode be treated as an undefined opcode so we can re-use it in the future, rather than defining it to have certain behavior. From the EIP it sounds like you have plans to research this further, so I think the results of that research will help inform whether we should do as this EIP suggests, or just free up that opcode and pretend it never existed.
It was definitely used in some experiments, that’s how DELEGATECALL emerged. I am still torn which is easier: a) having a stub implementation as proposed on EIP-2488, or b) having the current implementation.
But you are correct, more research is needed before concluding on a final proposal.