You may see this question as a logical successor of this post.
With the new Whatsapp Web update it is now usable without keeping the smartphone online. I can't imagine how this is compatible with End-to-End-encryption since the private key for decrypting messages has to be shared with the linked device through Whatsapps servers. In the FAQ for this feature Whatsapp just says "Your personal messages, media, and calls are end-to-end encrypted." but doesn't elaborate. The Encryption Whitepaper hasn't (yet) been adapted.
Does anyone know how this feature is (or could be) implemented and if this is still compatible with E2E-encryption?