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From BIP0342:

The opcode 186 (0xba) is named as OP_CHECKSIGADD.

What did 0xba do prior to this?

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Before the Taproot soft fork the opcode 186 (0xba) was an undefined opcode. Undefined opcodes abort verification and consider the transaction to be invalid. There are also OP_NOP opcodes that are designed to be ignored (do nothing) and do not invalidate the transaction. After the Taproot soft fork the opcode 186 (0xba) was defined as OP_CHECKSIGADD. There was an additional change in the Taproot soft fork where opcodes that remained undefined (not the case for the opcode 186) were made OP_SUCCESS. From BIP 342:

If any opcode numbered 80, 98, 126-129, 131-134, 137-138, 141-142, 149-153, 187-254 is encountered, validation succeeds (none of the rules below apply). This is true even if later bytes in the tapscript would fail to decode otherwise.

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    Thanks, I think there are also some specified OP_NOPs that do not invalidate the transaction.
    – Lee
    Commented Apr 26 at 9:05
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    @Lee: Indeed, I'm getting into a muddle here (been a while since I looked at this). There used to be undefined opcodes and OP_NOPs and after Taproot there are OP_SUCCESSes and OP_NOPs (I think) :) Commented Apr 26 at 9:15

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