The heart is of course protected by the rib cage, so no direct contact can be made to the heart, however as we all know any good punch to the rib cage does spoil your day for a while.
Holyfield targeted Lewis' chest and heart with jabs in their rematch and it was actually very effective.
It takes one's courage away. An old-school shot. When repositioning or framing, Foreman tended to occasionally strike at heart area.
Still seems like they don't really aim for it. An unspoken don't punch me in the tit and I won't do it to you.
Boxers chests are nowhere near as developed as the vast majority of athletes because their shoulders are so dominant over their chest. Sure, there's muscle, but it doesn't stop the pain of a shot to heart. Not at all. I'm pretty sure it's just a gentleman's agreement.