Microsoft Xbox’s division currently does not offer out-of-the-box support for keyboard and mouse devices for its Xbox One units, but there are different APIs that allow video game developers to do so with the use of 3rd party adapters.
This was recently confirmed in a Twitter conversaton by Mike Ybarra, Corporate Vice President of the Xbox Program Management at Microsoft. His response was directed to someone asking about keyboard and mouse support for FPS games.
There has always been much contention over the inclusion of these APIs as some gamers feel that using a keyboard and/or mouse peripheral could offer a better advantage over those playing with the regular Xbox One controllers. However, Ybarra made note that removing said APIs could prevent game developers from offering specialized controller options for gamers with accessibility issues.
However, it appears that keyboard and mouse support may eventually be officially supported in the future, even though no definitive guideline has been given as to when it will come to be. They are also considering including the option for players to be matched with the type of controllers they use so that one party does not have an unfair advantage over the other.