If you're on android, you can use an automation tool like Tasker to automatically forward your WhatsApp messages to other apps(or email) and even automatically reply to WhatsApp messages using the notifications from the WhatsApp app(Requires bit of learning curve and add-on purchases). Someone adventurous can set this up as cloud service using android VMs(e.g. anbox) and offer it to end-users.

But even if a bot uses WhatsApp on behalf of you for forwarding/replying to the WhatsApp messages it's still you as far the data is concerned and thereby defeating the purpose of why you might have wanted to leave WhatsApp in the first place. Although amount of data you give to WhatsApp/Facebook could probably be controlled.