The blue light filter is a feature that was first reported a few months ago, but we haven't heard much about it. It looks like the feature is almost ready, as it has appeared in the latest Windows 10 build 14997, which has leaked online today. The feature will probably find its ways into the next Windows 10 Insider build early next year, following the official announcement with Creators Update.
Those who installed the leaked beta build will find this new "Blue light settings" feature in the Settings app. They can also adjust the color temperature or schedule the filter to be turned on at a specific time.