YouTube has the advantage of being able to preprocess the audio track in its entirety, although most implementations work on a block basis and don't need that. A 'night mode' feature would take about one weekend's worth of work from start to finish.
You would also need a fairly sophisticated content detection system (don’t distort anything that is music, which is a huge use case on YouTube for example).
No, you need a button that the user can press to turn the feature on and off. It should default to 'off.' For extra points, make it a slider.
Not to mention adding _any_ steps to their intake pipeline requires more processing power and data moving than you and I will ever touch in our lifetimes.
Compared to the work needed to process the video itself, it would be a total non-issue.