The principles of Einstein-free physics are so obviously correct that even mainstream physicists unwittingly advocate them sometimes:
University of Illinois at Urbana-Champaign: "Consider a falling object. Its speed increases as it is falling. Hence, if we were to associate a frequency with that object the frequency should increase accordingly as it falls to earth. Because of the equivalence between gravitational and inertial mass, we should observe the same effect for light. So lets shine a light beam from the top of a very tall building. If we can measure the frequency shift as the light beam descends the building, we should be able to discern how gravity affects a falling light beam. This was done by Pound and Rebka in 1960. They shone a light from the top of the Jefferson tower at Harvard and measured the frequency shift. The frequency shift was tiny but in agreement with the theoretical prediction." https://courses.physics.illinois.edu/phys419/sp2011/lectures/Lecture13/L13r.html
Two principles implied in this particular scenario are actually valid in any scenario:
(1) Frequency and speed of light vary proportionally.
(2) The wavelength of light is invariable.
It is easy to see that (1) and (2) are equivalent, given the formula (frequency)=(speed of light)/(wavelength).
Another scenario where (1) and (2) are obviously true is Doppler (moving observer):
"Thus, the moving observer sees a wave possessing the same wavelength [...] but a different frequency [...] to that seen by the stationary observer." http://farside.ph.utexas.edu/teaching/315/Waveshtml/node41.html
"The wavelength is staying the same in this case." https://www.youtube.com/watch?v=MHepfIIsKcE
"Vo is the velocity of an observer moving towards the source. This velocity is independent of the motion of the source. Hence, the velocity of waves relative to the observer is c + Vo. [...] The motion of an observer does not alter the wavelength. The increase in frequency is a result of the observer encountering more wavelengths in a given time." http://a-levelphysicstutor.com/wav-doppler.php
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