What should Developers do if a new feature added to the Increment was not in the Product Backlog?

The most appropriate action for Developers when a new feature added to the Increment was not included in the Product Backlog is to remove it immediately if possible. This is aligned with the principles of Scrum, which emphasize transparency and the need for the Product Backlog to accurately reflect the requirements and desired features of the product.

When a feature is not part of the Product Backlog, it indicates that it has not been agreed upon by the Stakeholders or the Product Owner, and may not align with the priorities established for the product. Therefore, removing it ensures that the Increment remains consistent with the planned scope and allows the team to focus on delivering what has been agreed upon.

Additionally, adherence to the Product Backlog ensures that the Scrum Team maintains a clear and shared understanding of priorities, allowing for informed decision-making and effective communication with stakeholders. If a feature is included in the Increment without proper backlog documentation, it can disrupt the planned course of development and potentially lead to confusion or misalignment among the team and stakeholders regarding project goals.