Properly register client channels.

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

@@ -65,7 +65,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
   @Override
   public void activated() {
     PluginMessage register = PluginMessageUtil.constructChannelsPacket(player.getProtocolVersion(),
-        server.getChannelRegistrar().getModernChannelIds());
+        server.getChannelRegistrar().getChannelsForProtocol(player.getProtocolVersion()));
     player.getConnection().write(register);
   }
 

proxy/src/main/java/com/velocitypowered/proxy/util/VelocityChannelRegistrar.java

@@ -6,6 +6,7 @@ import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
 import com.velocitypowered.api.proxy.messages.ChannelRegistrar;
 import com.velocitypowered.api.proxy.messages.LegacyChannelIdentifier;
 import com.velocitypowered.api.proxy.messages.MinecraftChannelIdentifier;
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Map;
@@ -68,4 +69,11 @@ public class VelocityChannelRegistrar implements ChannelRegistrar {
   public @Nullable ChannelIdentifier getFromId(String id) {
     return identifierMap.get(id);
   }
+
+  public Collection<String> getChannelsForProtocol(int protocolVersion) {
+    if (protocolVersion >= ProtocolConstants.MINECRAFT_1_13) {
+      return getModernChannelIds();
+    }
+    return getIdsForLegacyConnections();
+  }
 }