Earlier quoted context omitted.
I would love to move to Signal and send anyone reaching out to me on Whatsapp an auto-reply with a Signal invite. Couldn't find how to achieve that. Anyone knowing if this is possible?
Facebook has an API for whatsapp, which I believe is a closed beta. In theory you could build an app to do this for you, twilio supports this and let's you use a development key even before you get into the beta. [I expect that's too many hoops for your use case though ;]
https://www.whatsapp.com/business/api
The Twilio API is in beta, and you can sign up. Don't know what pricing will be, but maybe for individual use a dev subscription may be sufficient
https://www.twilio.com/whatsapp
Here is the API reference: