Update at 5:22PM: It is now showing once again in the Xbox App. A confusing situation for sure!
Update as of 4:38PM May 5th, 2021: Psychonauts 2 is no longer available on the store or Xbox app. It has been hidden, if you have the game installed you can choose “show in the store” and the page comes up on the console itself. This could possibly mean a shadow drop at E3 is planned and a mistake to show it publicly occurred.
Psychonauts 2 has shown up on the Xbox Store, though it is unavailable for pre-order and no date is listed. You can however set the game to download through the Xbox App and it is showing up as a 27.7gb file on the Xbox Series X console.