You received the email because you haven't logged into your account in the last 365 days, and haven't used any paid resources with Heroku for the past six months. We will also delete any unpaid apps and add-ons that solely belong to an inactive account.
In addition to our plans for inactive account deletion, Heroku has also ended free Heroku Dynos, free Heroku Postgres and free Heroku Data for Redis plans starting November 28, 2022.
See our blog post and the Removal of Heroku Free Product Plans FAQ for more information.