In order to get the source for any event that is CauseTracked, you now must query for the Cause. So for example, lets assume you want to see if a Player caused a BreakBlockEvent, you would do the following :
public void onBreakBlockEvent(BreakBlockEvent event) {
Optional<Player> player = event.getCause().first(Player.class);
if (player.isPresent()) {
// do stuff
}
}