A key quote here:

"He (Moxie) said: “The choice to make these notifications ‘blocking’ would in some ways make things worse. That would leak information to the server about who has enabled safety number change notifications and who hasn’t, effectively telling the server who it could man-in-the-middle transparently and who it couldn’t; something that WhatsApp considered very carefully.”

This claim is false. Those “blocking” clients could instead retransmit a message of the same length that just contains garbage and this message would just not be displayed by the receiver’s phone. Encryption guarantees the garbage or real messages are indistinguishable in the encrypted form. Hence, this technique would make identifying users with the additional security enabled on a large scale impossible."

This was raised in the previous WhatsApp vuln thread but as far as I'm aware, Moxie is yet to address this criticism. Would be good to get a response on this.