Every once in a while I see some feature like app XYZ getting dark mode or spell check pop up on the HN front page. I don't understand why. These are absolutely basic features ever since the turn of the millennium, so why are these being announced/launched like some big deal?

Am I missing something here? Why is this considered a noteworthy feature when my operating system has provided this API enabled by default for all text and document views for over twenty years now? (https://leopard-adc.pepas.com/documentation/Cocoa/Conceptual...)