It was expressed clumsily. The idea would be that photons can lose energy over time or space. One problem is that they would all have to lose energy at exactly the same rate as neighboring photons, so be a property of the space they are going through, not the photons themselves.

The speed would remain constant; 'tired light' would shift to longer wavelengths (redden).

Fritz Zwicky proposed the idea in 1929 - didn't work out. In 1933 he 'inferred the existence' of dark matter/energy, after using the averaged speed of a group of galaxies to calculate their mass ... and found they were producing too little light to have that mass. And there it sits a century later. MON Dieu!

C is constant, which is the "speed of light in a vacuum." Light can, and does, travel slower than C, e.g. when traveling through gas or other material.

https://en.m.wikipedia.org/wiki/Speed_of_light

Scientists have even managed to freeze light for short periods of time.

https://io9.gizmodo.com/scientists-freeze-light-for-an-entir...

Constant in a vacuum. Read about https://en.wikipedia.org/wiki/Cherenkov_radiation

No. Gravitational waves are causing fluctuations of speed of light in pure vacuum. (See LIGO/VIRGO experiments). So c is constant in steady physical vacuum only, i.e. it's speed of EM wave in medium. If you will change properties of medium, then c will be changed too.