It depends on who is designing and who is manufacturing. People like Manolo Blahnik sketches his shoes (he hasn't the faintest idea how to actually make a pair of shoes...) and his designs are translated by shoe-makers.
Hand shoemakers of course design their own heels but they tend to rely on old favourites. However sometimes they develop a heel that is picked up by others like French shoemaker Charles Jordan (NOT JOURDAN) who developed the slim stiletto heel in the early 1950s but it took several seasons of copying by other designers to catch on. Perugia and Vivier were both heel innovators as well.
Manufacturers however read fashion forecasts and study sales trends to see what people are buying and copy their styles from other lines. The only new heel in the last ten years is the flat stiletto, which looks like a stiletto heel from the side but a block heel from the back because of its width. That came out about a decade ago and was still around last season when I checked. Before that the upside down triangle heel which came out in about 1979 but was the main style of the 1980s. Otherwise all heels are revivals of past styles -- cuban, louis, spanish, stiletto, block, wedge, etc. asa well as the occasional novelty heel style, like orbs, pineapples, Atlas holding the world etc. but these heels are usually only one season long in life.
RIght now manufacturers can not afford to push a main trend and must supply a variety of styles for the general audience so they produce everything at once. This developed during the 1990s when Louis heels and platforms came back in fashion. Many women chose not to wear those styles and the stiletto continued to be worn, even though they weren't being promoted as high fashion. Eventually consumer demand brought back the stiletto heel but platforms remained as well, so today we are living with multiplicity of fashion designs at once. It used to be that only one heel style in various heights was available but no more.
If you are buying designer shoes, like Christian Louboutin or Manolo Blahnik, the heels are designed by the designer, but if you are buying shoes from Nine West or whatever, you are probably buying a pair of shoes that no designer ever looked at. Rather the shoe was copied from existing pairs and tweaked for their own clients, probably using computer technology and not shoe makers.
