Multi-Signature – What did opcode 0xba do before generating OP_CHECKSIGADD?

Prior to the Taproot soft fork, opcode 186 (0xba) was an undefined opcode and was treated as OP_NOP. OP_NOP stops verification and considers the transaction invalid. Since the Taproot soft fork, opcode 186 (0xba) has been defined as OP_CHECKSIGADD. There was an additional change in the Taproot soft fork where opcodes left undefined (not for opcode 186) were made OP_SUCCESS. From BIP 342:

Validation succeeds if opcodes numbered 80, 98, 126-129, 131-134, 137-138, 141-142, 149-153, 187-254 are found (none of the rules below apply). This is true even if subsequent bytes in the tabscript would otherwise fail to be decoded.