Use IFTTT as a Heroku deploy hook.
First, go to the Maker channel on IFTTT and connect it.
Finally, go to any other Heroku app for which you wish to notify IFTTT when they're deployed, and add an HTTP deploy hook pointing to
https://[URL]/ifttt/[your Maker secret key], where
[URL] is the URL of the Heroku app you just created, and your Maker key is the one you can get from the Maker channel, like so:
heroku addons:add deployhooks:http --url=https://your-ifttt-heroku-app.herokuapp.com/ifttt/abc_123_whatever
IFTTT will get notified whenever the app is deployed, which you can use as a trigger when setting up IFTTT recipes. When creating a new recipe, use
heroku_deploy as the event name.
value1 is the notification text and
value2 is the URL of the app; you can use these values in the action channel, like this: