How is that difficult, expensive, or unreliable? What could be simpler than a switch that closes when a lever is closed, and turns on a light?

That sounds much simpler, cheaper, and reliable than "detect changes in the frequency of the wheel rotation".

Also, it works like a car's brake lights, which drivers are accustomed to, rather than "start pulsing when you decelerate", which is when you stop pedaling (unless going downhill).