This would allow for embedding in a Taproot script such as a Decker-Russell-Osuntokun with an internal Taproot point of 2-of-2. However, proposing a new opcode may lead to increased design space and potential bikeshedding. It is important to note that any special script would be supportable by Taproot and already targetable by a Taproot address. The advantage of using a sighash flag is that the spender can specify the flags while an opcode would dictate the signature verification modalities.