I have no insider knowledge, but a look thought their changelog[1] is not exactly inspiring, especially if you consider that Heroku doesn't support things like HTTP2.[2] There are almost no real changes beyond updates to languages and the occasional (rare) minor change to the platform, which is not a sign of a healthy actively supported service.

At work, we spend a LOT of money with heroku every month. We really wanted to use Protobuffers (please let's not discuss that ;) ), which require HTTP 2 for the streaming stuff. So we talked with them and were like "heyy, we need this" and while they didn't explicitly say they weren't going to support it, the response basically boiled down to "yeah, i wouldn't be holding your breath" ;)

You can use protobuff without gRPC if that helps for your use case. Twitch has open sourced their HTTP1 protobuff client/server library: https://github.com/twitchtv/twirp