The single/double checkmark is used with Signal's client as well. When you're sending unsecured texts, you get a single checkmark when you send it; when you are sending encrypted messages, a lock symbol and a single check when sent, and a double check when received.
Except that the obvious interpretation of the second checkmark not appearing is that the message was never received, not that it was decrypted by an attacker. Especially when there's a key change warning afterwards. I think this is still the correct interpretation in Signal proper, but who knows at this point?
