PlayerKickEvent.setLeaveMessage does not actually do anything; the message is ignored. (PlayerQuitEvent.setLeaveMessage does work, however).

To reproduce, create a plugin with the following:

 @EventHandler
 public void onKick(PlayerKickEvent e) {
  e.setLeaveMessage("... a leave message ...");
 }

... and then run /kick on a player. The normal "<player> left the game" message will be displayed instead of the custom "... a leave message ..." one.

I'm cleaning up some of the related parts of the code for a larger PR, so I'll end up fixing this within that soon; I'm just creating this ticket as a reference until then.