I am being asked for heroku app id (the [email protected]) by 3rd party support. I am quite sure there was some way to do get it, but I don't see it know. It's not featured in heroku info even in the more verbose --json mode.
How can I retrieve it?

I had the same issue, and was informed via my support ticket that the only way to get the heroku app ID (that SendGrid uses) is to ask for it via Heroku support. Here's what Heroku support says:
Lodge a ticket here: https://help.heroku.com/?t=true
UPDATE
Turns out you can access it by running