If you downgrade your github account you don't get to keep the extra repos above the level you downgrade to.

Heroku offers metered billing and a free account. So does MailChimp. You weren't using either.

Yes, Github only removes access to your repositories. And some from my list allow a downgrade if enough projects are archived, not deleted, so they are still available. Still, it's not too common yet.

Heroku is different because they don't charge you to host code with them; they charge for number of dynos which only makes your app faster or slower. However, I don't think you can downgrade a large production database to free and keep it, or downgrade all of the add-ons to free.