The new default stack for Heroku apps is heroku-18, which deprecates support for Ruby 2.3 and under. This stack will be used for all new apps that do not have a
stack defined in their
app.json, which includes CI runs.
If you need to keep your CI runs on the
heroku-16 or earlier stacks, you can do so by defining this in your
app.json. Adding the following line will pin the stack for all future CI runs: