There’s a video where Craig is sitting on stage with Jimmy and he’s giving one example of why threshold is better than existing approaches. He asserts that external parties can see which of the co-signers signed off on the payment. This is an example of the laundry list of misconceptions about multi-signature payments in the real world.

What he does not know about, is the sorting of public keys. That in every single P2SH redeem script in the scriptsig the position of public keys that are signed is meaningless in terms of external visibility into which co-signer signed off on the payment.

This is as you essentially point out, a side-effect of the requirements of the deterministic P2SH scripthash. However, as there are at least two other benefits over that requirement, it has been retained for bare multisig in ElectrumSV, and will be more than likely retained for accumulator multisig. It is an imagined problem that has never really existed, not in Electrum, Electron Cash nor ElectrumSV. Not even in Copay which is supposed to have compatible multi-signature from what I understand.

So to restate as clearly as possible, people have never been able to tell which co-signer signed off on a payment and they won’t be able to in future, at least not in ElectrumSV. It’s an imagined problem, and a concrete example of the poor quality of the current promotions of threshold multi-signature.