@Zanaeira this endpoint was specifically written for the aladhan app almost 10 years ago and is called every 5 mins or so to get the next prayer time displayed at the bottom right on the /play page - so the caching doesn't really impact the experience on he web app. It was never part of the documentation until it was added to the OpenAPI specs earlier this year.
All endpoints initially only accepted timestamps, but that basically hindered the possibility of caching results and processing 3k requests per second without caching was not really feasible in the long run, so all endpoints were converted to dates.
Now, there are use cases where caching would definitely pose challenges for such an endpoint, and using a timestamp might as well mean there is no need for caching.
Even removing the endpoint from the documentation is an option, but before we do that or see what changes we can make to it, this is a calculation that than completely take place on the client side. Is there a reason why you aren't just doing this offline?
