Alberto Martinez https://liberalarts.utexas.edu/history/faculty/aam829: "Does the speed of light depend on the speed of its source? Before formulating his theory of special relativity, Albert Einstein spent a few years trying to formulate a theory in which the speed of light depends on its source, just like all material projectiles. Likewise, Walter Ritz outlined such a theory, where none of the peculiar effects of Einstein's relativity would hold. By 1913 most physicists abandoned such efforts, accepting the postulate of the constancy of the speed of light. Yet five decades later all the evidence that had been said to prove that the speed of light is independent of its source had been found to be defective." https://www.martinezwritings.com/m/Relativity.html

Martinez's words are euphemistic. The speed of light does depend on the speed of the source

https://qph.cf2.quoracdn.net/main-qimg-f10f1c25528a4e5edc9bae200640f31c-pjlq

as unequivocally proved by the Michelson-Morley experiment:

"Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment of 1887...The name most often associated with emission theory is Isaac Newton. In his corpuscular theory Newton visualized light "corpuscles" being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v)." https://en.wikipedia.org/wiki/Emission_theory

Banesh Hoffmann, Einstein's co-author, admits that, originally ("without recourse to contracting lengths, local time, or Lorentz transformations"), the Michelson-Morley experiment was compatible with Newton's variable speed of light, c'=c±v, and incompatible with the constant speed of light, c'=c:

"Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether." Banesh Hoffmann, Relativity and Its Roots, p.92 https://www.amazon.com/Relativity-Its-Roots-Banesh-Hoffmann/dp/0486406768

Another euphemistic (but still very instructive) text:

Alberto Martinez https://liberalarts.utexas.edu/history/faculty/aam829: "The story of Walter Ritz’s foray into electrodynamics is instructive for several reasons. Ritz was not one of the elder physicists who objected to Einstein’s theory in favor of more traditional approaches; he was young and regarded his approach to electrodynamics as far more radical than Einstein’s. Moreover, at first Ritz received more appreciation and support from the established physics community than Einstein. But Ritz’s prolific labors ended abruptly when he died in 1909 at the age of 31...In sum, Einstein rejected the emission hypothesis prior to 1905 not because of any direct empirical evidence against it, but because it seemed to involve too many theoretical and mathematical complications. By contrast, Ritz was impressed by the lack of empirical evidence against the emission hypothesis, and he was not deterred by the mathematical difficulties it involved. It seemed to Ritz far more reasonable to assume, in the interest of the "economy" of scientific concepts, that the speed of light depends on the speed of its source, like any other projectile, rather than to assume or believe, with Einstein, that its speed is independent of the motion of its source even though it is not a wave in a medium; that nothing can go faster than light; that the length and mass of any body varies with its velocity; that there exist no rigid bodies; that duration and simultaneity are relative concepts; that the basic parallelogram law for the addition of velocities is not exactly valid; and so forth. Ritz commented that "it is a curious thing, worthy of remark, that only a few years ago one would have thought it sufficient to refute a theory to show that it entails even one or another of these consequences."...Two months after Ritz's death, in September 1909, his exchange with Einstein barely echoed at a meeting of the Deutsche Naturforscher und Ärtze in Salzburg, where Einstein delivered a lecture elaborating his views on the radiation problem but made no explicit reference to Ritz's views. Two years later, however, in November 1911, Paul Ehrenfest wrote a paper comparing Einstein's views on light propagation with those of Ritz. Ehrenfest noted that although both approaches involved a particulate description of light, Ritz's theory constituted a "real" emission theory (in the Newtonian sense), while Einstein's was more akin to the ether conception since it postulated that the velocity of light is independent of the velocity of its source...Ritz's emission theory garnered hardly any supporters, at least none who would develop it or express support for it in print. As noted above, in 1911, two years after Ritz's death, Ehrenfest wrote a paper contrasting Ritz's and Einstein's theories, to which Einstein responded in several letters, trying in vain to convince him that the emission hypothesis should be rejected. Then Ehrenfest became Lorentz's successor at Leiden, and in his inaugural lecture in December 1912, he argued dramatically for the need to decide between Lorentz's and Einstein's theories, on the one hand, and Ritz's on the other. After 1913, however, Ehrenfest no longer advocated Ritz's theory. Ehrenfest and Ritz had been close friends since their student days, Ehrenfest having admired Ritz immensely as his superior in physics and mathematics; but following Ritz's death, Einstein came to play that role, as he and Ehrenfest became close friends." https://www.academia.edu/69646000/Ritz_Einstein_and_the_Emission_Hypothesis

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Why did Einstein win against Walther Ritz? Because the important theoreticians at that time (Poincaré, Lorentz etc) were all etherists, and Ritz's "speed of light dependent on the speed of the source" threatened to ruin their works and their careers. In contrast, Einstein was a conformist:

Albert Einstein: "I introduced the principle of the constancy of the velocity of light, which I borrowed from H. A. Lorentz's theory of the stationary luminiferous ether." https://en.wikipedia.org/wiki/Lorentz_ether_theory

So Poincaré & Co found themselves in a very awkward situation. They did not want to accept the preposterous conclusions of Einstein's special relativity, but at the same time they saw nothing wrong with the premises (postulates) from which these conclusions had allegedly been deduced. The reactions were different. Lorentz decided to share Einstein's fame, Poincaré remained critical but died prematurely, three years after Ritz (so in 1912 lucky Einstein had no serious opponents):

"From his [Poincaré's] point of view, the true velocity of light in a moving frame was not a constant but was given by the Galilean law of addition of velocities." https://www.jstor.org/stable/10.1086/430652

Henri Poincaré: "Lorentz could have accounted for the facts by supposing that the velocity of light is greater in the direction of the earth's motion than in the perpendicular direction. He preferred to admit that the velocity is the same in the two directions, but that bodies are smaller in the former than in the latter." http://www.marxists.org/reference/subject/philosophy/works/fr/poincare.htm

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Richard Feynman: "I want to emphasize that light comes in this form - particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you probably learned something about light behaving like waves. I'm telling you the way it does behave - like particles. You might say that it's just the photomultiplier that detects light as particles, but no, every instrument that has been designed to be sensitive enough to detect weak light has always ended up discovering the same thing: light is made of particles." QED: The Strange Theory of Light and Matter p. 15 https://www.amazon.com/QED-Strange-Theory-Light-Matter/dp/0691024170

Whether Feynman is correct is not a matter of discussion here. I am just drawing the attention to a crucial implication. The concept of VARIABLE wavelength of light

https://www.youtube.com/watch?v=xsVxC_NR64M

is preposterous if "light is made of particles". That is, the particle model of light implies that the wavelength can only be an invariable proportionality factor in the formula

(speed of light) = (wavelength)(frequency)

Elsewhere I have shown that VARIABLE wavelength of light is incompatible with the wave model of light as well. The wavelength of light depends only on the nature of the emitting substance and is constant otherwise.

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