Microsoft has provided mouse and keyboard support for both Xbox One and Alpha Ring members. This should arrive in November 2018 for everyone as it is part of the currently-tested system update "1811".
But slowly: No mouse or keyboard is used in any Xbox One game in November. Instead, Microsoft says it will work here:
With this new platform-level accessibility, developers can now create mouse and keyboard support in their games, if and how they choose. It's important to note that mouse and keyboard support for games is added title by title, at the designer's discretion. Nothing changes for all other titles. Mouse and keyboard input is not enabled by default for games. Each development team knows its titles best, and we help them create the right experience for their games to ensure an optimal and fair gaming experience.
Developers might need controllers in multiplayer games, or they could vote for the interaction of mouse and keyboard players with PC players online thanks to the Xbox One's support. It's up to every developer of the game. Warframe is the first game to test mouse and keyboard support.
Microsoft has announced that the Xbox One supports "most wired or wireless USB keyboards and mice." Currently, the Xbox One offers limited support USB keyboards, but only for input text. This feature was added in February 201
Sony already offers games with PlayStation and mouse, but only a few games support this input method. We'll see if Xbox One game developers are more interested in mouse and keyboard input.
Microsoft also teases Razer mice and keyboards designed exclusively for the Xbox and announced on November 10 at the Microsoft X018 event Keyboard support is also announced on X018, so stay tuned for more
Thanks to Thurrott for recognizing this news.