After a review of the steps needed to manage fraud and abuse from free resources on Heroku, we have made the choice to end free product plan offerings that will allow Heroku to focus on customers of all sizes. This decision includes deleting inactive accounts that only use free resources. Our blog post has more insight into this decision.
See our blog post and the Removal of Heroku Free Product Plans FAQ for more info related to the ending of free plans and inactive account deletion.