If you go looking for Kodi in the Microsoft Store right now expecting to be able to install it on an Xbox One, you'll be coming up short. The app has gone, and even if you previously had it installed on your Xbox you won't be able to re-install it as it's gone from the ready to install list as well.
Is this bad? Yes, because Kodi is a big deal for the Xbox. But right now at least it doesn't look like there are any nefarious reasons behind it.
One of the Kodi team has responded to the numerous reports in the official forums:
We are aware of the problem which has resulted in the disappearance of the XBox version of Kodi from the Microsoft Store. Members of the Team are currently working on resolving the issue. We appreciate your patience and we should have it visible in the Store soon.
It turns out that it disappeared a few days ago, but if you have it installed it will keep on working as fine as it ever did. We just noticed today having had to uninstall after breaking something that it wasn't there to put back on our Xbox. There doesn't appear to be an ETA on its return, either, so for now, make sure you don't uninstall it if you're actively using it.