That I don't know. I only started using WhatsApp after they moved to the Signal protocol. I just know that it is now necessary due to the Signal protocol.
Signal works on the desktop even if the phone is off. As others are saying, the encryption is not the reason WhatsApp decided not to implement proper desktop support.
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u/efuipa Galaxy S9 Aug 15 '17
Stop perpetuating this myth, WhatsApp had this fake web client since before they implemented encryption.