I'm not an expert here, but a lot of aerodynamic improvements were banned, and disc brakes were banned until they weren't. At an extreme, recumbents. Triathalon

I'm not an expert here, but a lot of aerodynamic improvements were banned, and disc brakes were banned until they weren't. At an extreme, recumbents. Triathalon handlebars in mass-start stages. There are minimum weights. In track racing, the bikes are required to have fixed gearing. The earliest Tour of France required riders to be self supporting, meaning they had to carry their own spare tires etc. One thing I've observed is that for us commuters (me too), bikes are no longer tied to racing. We can have anything we want. The ubiquitous "hybrid" is used in no racing discipline, and the classic mountain bikes that are favored by many computers are largely obsolete for competitive mountain biking.