Update 10/30: It seems that the delisted games have been restored.
Original story follows.
As the popularity of the Xbox Series X and Xbox Series S consoles grow in tandem with the overwhelming success of Xbox Game Pass, the Xbox storefront's library of titles is expanding quickly. What doesn't typically happen, however, is that games are removed from the Xbox storefront. After all, once a purchase has been made, buyers expect the Xbox store to make it accessible for download in perpetuity. That's why it's quite surprising that 18 games have suddenly disappeared from the Xbox storefront with no notice.
While it isn't clear if all 18 games disappeared at the same time, they're nonetheless absent now. And by removed, it means that they've been completely taken down from the Xbox Store. As in, even those who have purchased these games before no longer have the ability to find and download them. Buyers who have previously downloaded the games and still have them on their Xbox will still have them, of course, but if they delete them they'll no longer be accessible.
The 18 games in question aren't major releases, by any means. No one's going to wake up and wonder where Fortnite is; this isn't the iOS debacle all over again. As reported by TrueAchievements, the 18 games in question include: Cubikolor, Defender's Quest: Valley of the Forgotten DX, Electronic Super Joy, Frizzy, Gear Gauntlet, Gemini Heroes Reborn (DE), Gemini: Heroes Reborn, JumpJet, Rex, Marble Void, Mekazoo, Monkey Pirates, Mystery Castle, Past Cure, Pix the Cat, Q*bert REBOOTED: The XBOX One @!#?@! Edition, SlashDash, Tetrobot and Co, and The Station.
It should be clear how odd of an assortment of games the delisted titles are, especially when the most familiar name of the bunch is Q*bert. There doesn't appear to be any connecting trait between the games, outside of them all being smaller if not indie projects. All of them come from different publishers or are self-published on Xbox by small teams.
The simplest, and thus most likely theory is that the delisting occurred due to a technical issue or human error. However, the limited number of games and the fact that they're largely all small/indie projects implies something more. With that in mind, it's possible that there are licensing issues or perhaps fees that led to the removals, or that Xbox had to remove them for security flaws that hadn't been fixed due to the dev teams not being able to afford going back to the projects.
Whatever the reason, these 18 games have been removed from the Xbox storefront and there's no evidence that they're coming back anytime soon. Delisted Xbox games have returned in the past, however. Hopefully, those that play these games and enjoy them still have them installed. Otherwise, they're hopefully available on other platforms for archival purposes.
Source: True Achievements