1.8.8 Reactive unknown-command message

not possible with default implementation. if you wanted to add this, youd have to override SimpleCommandMap#dispatch(CommandSender, String)
from SimpleCommandMap:

Code (Java):

  public boolean dispatch(@NotNull CommandSender sender, @NotNull String commandLine) throws CommandException {
    String[] args = commandLine.split(" ");
    if (args.length == 0)
      return false;
    String sentCommandLabel = args[0].toLowerCase(Locale.ENGLISH);
    Command target = getCommand(sentCommandLabel);
    if (target == null)
      return false;
    try {
      target.timings.startTiming();
      target.execute(sender, sentCommandLabel, Arrays.<String>copyOfRange(args, 1, args.length));
      target.timings.stopTiming();
    } catch (CommandException ex) {
      target.timings.stopTiming();
      throw ex;
    } catch (Throwable ex) {
      target.timings.stopTiming();
      throw new CommandException("Unhandled exception executing '" + commandLine + "' in " + target, ex);
    }
    return true;
  }

when this returns false, the unknown command message gets printed.
from CraftServer (which would be hard to override)

Code (Java):

  public boolean dispatchCommand(CommandSender sender, String commandLine) {
    Validate.notNull(sender, "Sender cannot be null");
    Validate.notNull(commandLine, "CommandLine cannot be null");
    AsyncCatcher.catchOp("command dispatch");
    if (this.commandMap.dispatch(sender, commandLine))
      return true;
    if (!SpigotConfig.unknownCommandMessage.isEmpty())
      sender.sendMessage(SpigotConfig.unknownCommandMessage);
    return false;
  }

as you can see the functionality isnt there.

handle the unknown command in SimpleCommandMap#dispatch(CommandSender, String) but return false anyway and set the unknownCommandMessage to empty so it doesnt get printed on top of your custo message