Try to unbreak ViaVersion a little bit.

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -658,17 +658,13 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
   public boolean canForwardPluginMessage(ProtocolVersion version, PluginMessage message) {
     boolean minecraftOrFmlMessage;
 
-    // We should _always_ pass on new channels the server wishes to register (or unregister) with
-    // us.
-    if (PluginMessageUtil.isRegister(message) || PluginMessageUtil.isUnregister(message)) {
-      return true;
-    }
-
     // By default, all internal Minecraft and Forge channels are forwarded from the server.
     if (version.compareTo(ProtocolVersion.MINECRAFT_1_12_2) <= 0) {
       String channel = message.getChannel();
-      minecraftOrFmlMessage = channel.startsWith("MC|") || channel
-              .startsWith(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL);
+      minecraftOrFmlMessage = channel.startsWith("MC|")
+          || channel.startsWith(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL)
+          || PluginMessageUtil.isLegacyRegister(message)
+          || PluginMessageUtil.isLegacyUnregister(message);
     } else {
       minecraftOrFmlMessage = message.getChannel().startsWith("minecraft:");
     }

proxy/src/main/java/com/velocitypowered/proxy/protocol/util/PluginMessageUtil.java

@@ -61,6 +61,27 @@ public class PluginMessageUtil {
         .equals(UNREGISTER_CHANNEL);
   }
 
+  /**
+   * Determines whether or not this plugin message is a legacy (<1.13) registration plugin message.
+   * @param message the plugin message
+   * @return whether this is a legacy register message
+   */
+  public static boolean isLegacyRegister(PluginMessage message) {
+    checkNotNull(message, "message");
+    return message.getChannel().equals(REGISTER_CHANNEL_LEGACY);
+  }
+
+  /**
+   * Determines whether or not this plugin message is a legacy (<1.13) unregistration plugin
+   * message.
+   * @param message the plugin message
+   * @return whether this is a legacy unregister message
+   */
+  public static boolean isLegacyUnregister(PluginMessage message) {
+    checkNotNull(message, "message");
+    return message.getChannel().equals(UNREGISTER_CHANNEL_LEGACY);
+  }
+
   /**
    * Fetches all the channels in a register or unregister plugin message.
    * @param message the message to get the channels from