Added login listener to prevent players from joining with an unsupported version

src/main/java/net/krakatoa/proxy/ProxySystem.java

@@ -12,6 +12,7 @@ import lombok.SneakyThrows;
 import net.krakatoa.proxy.command.CodeCommand;
 import net.krakatoa.proxy.command.SendTestMessageCommand;
 import net.krakatoa.proxy.config.ConfigHandler;
+import net.krakatoa.proxy.listener.LoginListener;
 import net.krakatoa.proxy.listener.PlayerDisconnectListener;
 import net.krakatoa.proxy.listener.PostLoginListener;
 import net.krakatoa.proxy.listener.SocketClientMessageListener;
@@ -83,7 +84,7 @@ public class ProxySystem extends Plugin {
 
   public void loadListeners() {
     PluginManager pluginManager = getProxy().getPluginManager();
-    Arrays.asList(new PostLoginListener(), new PlayerDisconnectListener())
+    Arrays.asList(new PostLoginListener(), new PlayerDisconnectListener(), new LoginListener())
         .forEach(listener -> {
           pluginManager.registerListener(this, listener);
         });

src/main/java/net/krakatoa/proxy/listener/LoginListener.java

@@ -0,0 +1,28 @@
+package net.krakatoa.proxy.listener;
+
+import net.krakatoa.proxy.ProxySystem;
+import net.md_5.bungee.api.ChatColor;
+import net.md_5.bungee.api.chat.BaseComponent;
+import net.md_5.bungee.api.chat.ComponentBuilder;
+import net.md_5.bungee.api.event.LoginEvent;
+import net.md_5.bungee.api.plugin.Listener;
+import net.md_5.bungee.event.EventHandler;
+
+public class LoginListener implements Listener {
+
+  // https://wiki.vg/Protocol_version_numbers
+
+  private final BaseComponent[] component = new ComponentBuilder(
+      "You must use 1.18.X to play here").color(
+      ChatColor.RED).create();
+
+  @EventHandler
+  public void onLoginEvent(LoginEvent event) {
+    ProxySystem.getInstance().getLogger().info("Version: " + event.getConnection().getVersion());
+
+    if (event.getConnection().getVersion() < 757) {
+      event.setCancelled(true);
+      event.setCancelReason(component);
+    }
+  }
+}