We currently use CircleCI to deploy branch specific CloudFormation stacks for all branches with open PRs. This gives us the flexibility to develop features using isolated AWS resources and allows us to test our infrastructure along with our code. For branch stack deployments and updates this workflow works very well, a new branch stands up a new stack and any changes to that branch update the associated CloudFormation stack. The issue is that there is currently no way to clean up these stacks after a branch is closed and the PR is merged or rejected. For us this is currently a manual process.
It would be great if CircleCI exposed workflow triggers for more GitHub events. Specifically we would really benefit from a workflow that was triggered by a PR closed event or a branch delete event.