I couldn't find any calls of update_queue.task_done() in the codebase.

You can use the following handler to demonstrate the problem - it returns incrementing numbers:

def echo(bot, update):
    reply_text = f"Unfinished tasks: {bot.dispatcher.update_queue.unfinished_tasks}"
    bot.send_message(chat_id=update.message.chat_id, text=reply_text)

I personally would really like to know whether my bot is resting and all jobs are completed. It could help me to properly exit from a serverless handler.

Is there any reason why task_done() is never called? If there isn't, what would be the best place to put that logic in? Thanks.