SPIGOT-5810, SPIGOT-5835: 'Better' handling of Player.isOnGround

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/entity/Player.java

@@ -163,6 +163,20 @@ public interface Player extends HumanEntity, Conversable, OfflinePlayer, PluginM
      */
     public boolean performCommand(@NotNull String command);
 
+    /**
+     * Returns true if the entity is supported by a block.
+     *
+     * This value is a state updated by the client after each movement.
+     *
+     * @return True if entity is on ground.
+     * @deprecated This value is controlled only by the client and is therefore
+     * unreliable and vulnerable to spoofing and/or desync depending on the
+     * context/time which it is accessed
+     */
+    @Override
+    @Deprecated
+    public boolean isOnGround();
+
     /**
      * Returns if the player is in sneak mode
      *