Are the captain's and first officer's brake pedals always mechanically linked? I heard that for Boeing and Airbus aircraft, the pilot's and copilot's brake pedals are mechanically linked, is that true?

Why did they make that design decision? What's the advantage? Do all kinds of passenger aircraft have the same design?
It is cost effective for one. With an unlinked system, you now need to resolve disagreement between inputs some other way. Software has its own challenges. It's also instant, non-verbal communication between captain and first officer. A pedal moves and the other's pedal has already moved. No confusion or pilot effort to communicate it.