The proposed Taproot soft fork (BIP 342) defines the new opcode OP_CHECKSIGADD to verify Schnorr multisignatures whilst OP_CHECKMULTISIG (and OP_CHECKMULTISIGVERIFY) are disabled for Taproot spends (SegWit version 1). Could we use the OP_CHECKSIGADD opcode for verifying new SegWit version 0 ECDSA multisignatures? Obviously you don’t get the same batch verification benefits if the individual signatures are ECDSA but it would still be more efficient to use the counter system of CHECKSIGADD?
"The opcode 186 (0xba) is named as OP_CHECKSIGADD"
"OP_CHECKSIGADD is functionally equivalent to OP_ROT OP_SWAP OP_CHECKSIG OP_ADD"