The muted do not know (but may suspect) that another member has muted them. But they can't be certain unless that member tells them. Either in a childish get-in-the-last-word of "muted" or something similar. The mute is just a single click way to enforce a "you know what, I am never replying to this person again" desire. People always want to "get the last word in" or "win the argument", as if such a thing is/was possible. So no, if you've muted someone, or someone has muted you, the only way that the muted person will know is if they are told by the person who clicked on the button.