Xbox One will be receiving full mouse & keyboard support in the next few weeks, although the exact date for the feature's launch has not been stated yet. Microsoft partnered up with Razer for the launch, but other peripherals will work too.
Mouse and keyboard support will be coming to the members of Xbox Inside program first, who are pretty much the testing squad for the new features regarding Xbox One. Therefore, the addition of mouse and keyboard support will first go through an unofficial closed beta, after which it should be made available for wider audience.
Microsoft will announce existing games that will support this input method on 10 November 2018 and talk about future games that will also support it. One can't help but wonder if Fortnite will be on that list, which is Microsoft's flagship for the "we love cross-play motto". Should Xbox One players get their hands on keyboard+mouse input, they would easily find themselves at an advantage over Nintendo Switch cross-users, due to the expanded input options and precision.
That said, according to Microsoft the developers will now be able to decide for themselves whether and how they want to implement this input method to Xbox One titles. One of the pioneers in this area will be Digital Extremes with Warframe. Thankfully, their wildly successful title focuses on co-operative play so input advantage shouldn't harm the player base in this case.
As stated previously, Microsoft partnered up with Razer for the launch of this feature, but that doesn't mean players will have to sell their kidney in order to get the peripherals they want to use. According to the official post, "most wired or wireless USB keyboards and mice will work on Xbox One".
Xbox Insider that will be hosted on 10 November will talk a bit more in-depth about the partnership with Razer and whether there will be promotions for using or purchasing their gear.
Microsoft have also asked Xbox One users to provide feedback from using mouse and keyboard on the platform in order to ensure the best possible performance. The best possible way to do so at the moment is through Xbox Insider program.