I still prefer Threema. Its server is located in Switzerland, where no Patriot Act applies, and, what’s more, WhatsApp will still collect your user data (and merge them with Facebook’s collection of user data).
I‘ve looked into this today. The API uses the Threema Gateway which requires a (pre-)paid subscription, and it‘s not cheap. See Our Price Plans for Your Needs – Threema
I honestly doubt there would be many OpenHAB users willing to pay these prices for Threema notifications.
Thank you for your message and your interest in Threema Gateway.
In order to send Threema messages from your own environment, some development effort needs
to be done on your part. You can find the technical documentation for your developers
here: API - Threema Gateway. An access to Threema Gateway via the
app is unfortunately not possible.