You can send an instant message. That's instant. It does not mean the recipient must, or even should feel that they must, reply immediately. You won't know if the other person has read your message and decided to reply or not. That's not synchronous at all.

A voice conversation, on the other hand, is synchronous. You know when the other person has decided to leave the conversation or simply to not reply. That is synchronous.

> You may argue it has changed, but it is false to dispute the original intent.

I am not arguing it has changed. I am stating that it simply isn't what you claim it to be. That might be how you use it but that doesn't mean that everyone should feel obligated to.