No, Isaac Newton did not prove that the speed of light was constant. In fact, Newton's theory of light, known as the corpuscular theory, proposed that light consists of particles (corpuscles) that travel in straight lines and have different speeds depending on the medium through which they propagate.
It was Albert Einstein who later proposed that the speed of light in a vacuum is constant and independent of the observer's motion. This postulate is one of the fundamental principles of his theory of special relativity, which he introduced in 1905. According to Einstein's theory, the speed of light in a vacuum, denoted by "c," is approximately 299,792,458 meters per second, and this value is the same for all observers, regardless of their relative motion.
Einstein's assertion of the constancy of the speed of light was based on theoretical considerations and experimental evidence. He recognized that Maxwell's equations, which describe the behavior of electromagnetic waves, implied a constant speed of light. Additionally, the Michelson-Morley experiment conducted in 1887, which aimed to detect the motion of the Earth through the hypothetical "luminiferous ether," failed to detect any such motion, further suggesting that the speed of light is constant.
Einstein's theory of special relativity provided a consistent framework that explained the constancy of the speed of light and its implications for space, time, and the nature of motion. It revolutionized our understanding of physics and had significant implications for many areas of science and technology.