This is important.

src/main/java/com/gmail/nossr50/listeners/PlayerListener.java

@@ -68,7 +68,7 @@ public class PlayerListener implements Listener {
             return;
         }
 
-        deathMessage.replaceAll("(?:\u00A7(?:[0-9A-FK-ORa-fk-or]){1}(?:[\u2764\u25A0]{1,10})){1,2}", "a mob");
+        deathMessage = deathMessage.replaceAll("(?:\u00A7(?:[0-9A-FK-ORa-fk-or]){1}(?:[\u2764\u25A0]{1,10})){1,2}", "a mob");
         event.setDeathMessage(deathMessage);
     }