An observer undergoing proper acceleration experiences physical acceleration that can be detected locally, such as via an accelerometer, which is a foundational concept in relativity.
The claim is a foundational and uncontroversial physical principle: proper acceleration is an invariant quantity that an observer physically feels (e.g., via the force of a seat pushing against them or an accelerometer reading). None of the retrieved papers directly test or challenge this textbook definition, and the concept itself is universally accepted as a direct consequence of spacetime kinematics.