Drone automatically executes pipelines for all event types (pull request, push and tag) unless you explicitly configure it to include or exclude a subset of events. For example, if you only want to execute a pipeline for push events it must be explicitly configured in the trigger section like this:
or like this:
This means if we add support for github_pull_request_review events, for many existing projects, pipelines would begin to automatically execute for github_pull_request_review events. The sudden execution of pipelines for a new event type would be unexpected and undesired for most of our users, which is why we would consider this a breaking change.
It is probably fair to say that we may need to rethink how we trigger pipelines, however, that would likely lead to breaking changes as well. This is definitely something we can consider for the future, when we are ready to make a breaking release.