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.
[1]: https://devcenter.heroku.com/changelog
[2]: https://help.heroku.com/JAOCNZ25/does-heroku-have-plans-to-s...