Froogo

Sorry, meant if the end user's network was compromised, not the server's.

Whether or not you're on top of your game, HTTP leaves you vulnerable to MITM attacks[0]. So this would leave the end user vulnerable to any of these attacks even assuming Heroku has everything else perfect.

[0] https://en.wikipedia.org/wiki/Man-in-the-middle_attack

I guess a concern could be that since this URL is insecure, it could redirect you wherever assuming your network is compromised (either their malicious URL, or even HTTP for the legit Heroku site assuming you don't have HSTS on it).

Obviously, you've checked the URL and seen that it's legit after, but realistically you should expect a legit email to not be feeding you a potentially insecure link.