The new one has circuitry to identify it individually and associate it to your Apple ID. In addition to giving notifications when detached from your phone, you can also set the wallet to display your phone number if someone else finds it and attaches it to their phone. Pretty sure some sort of unique identifier is needed in the wallet so the phone knows which wallet has been attached or detached.
I do agree that the detach notifications are completely software driven and should be enabled for the old wallets. But I don’t think there is any way for the phone to know that it was your specific wallet that was connected without whatever chips is in the new wallets.
The difference is the new one has to have some sort of unique identifier for each specific wallet since it has the ability to display your phone number if someone else attaches it to their phone.
I don’t think so…. The Find My trick apparently requires new hardware. Apple discontinued its previous clip-on wallet on the dash of the announcement of the new one, and is only offering the new one. There’s no reason to do that if the new and old versions are functionally the same.