Hmm ... so the heart plexus is an internal structure protected by the rib cage? Not so likely to produce a lethal result via an unarmed strike in that case. That would be a legal target not only in MMA, but most full contact striking competition - including boxing. Muay Thai, Kyokushin Karate, etc. I don't think I can recall a single fatality due to a strike to that location. The brachial plexus is a different spot and a much better target for striking. It's unlikely to have lethal results, but it can produce knockouts. Some combat sports allow strikes to the brachial plexus while others do not. MMA allows it.