I use to measure the poisonous chlorine and flouride added to the water to make sure it's levels were high enough. I had access to chemical measuring equipment where I worked. I took samples of water after going through my KDF shower filter several years ago and guess what? No chlorine. And check this out, the filter was well over 30 days old. It was probably closer to 6 months old.
The KDF filter is an ion-exchange. The filter prefers chlorine, and it exchanges it for something else---That goes in the shower water.
This was expensive equipment, not just the drops the change color in the presence of chlorine.