You may be pondering of legitimate but non-standard transactions. From the perspective of the node software program:
Standardness is what kind of transactions ought to be included.
Validity is what kind of transactions might be included.
If a node sees a potential transaction as non-standard it is not going to share it on the P2P community and won’t embody it in candidate blocks for mining, even whether it is legitimate. Nonetheless, if it sees non-standard but legitimate transaction has been confirmed on the chain, it can settle for it.
Bitcoin Core and and Bitcoin Knots have a barely totally different perspective (coverage) on standardness, however agree on validity. If nodes have been to disagree on validity, there would seemingly be a fork.
…how will or not it’s backwards suitable?
It may be value taking a look at OP_CHECKSIGADD for an instance opcode solely utilized in Tapscript.
case OP_CHECKSIGADD:
{
// OP_CHECKSIGADD is simply obtainable in Tapscript
if (sigversion == SigVersion::BASE || sigversion == SigVersion::WITNESS_V0) return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
But when it’s a coverage, op_cat can seem at this time with a assist of a miner?
No, the script could be invalid. Such a transaction, and its block, could be rejected as invalid by all nodes working a Bitcoin Core model, or derivation, launched since OP_CAT was eliminated (i.e. ~all of them).
