Mace no longer available after purchasing from MW store

Hi here is the guide from the Xbox Support website about troubleshooting Add-ons.

https://beta.support.xbox.com/help/games-apps/my-games-apps/view-downloadable-content

Troubleshooting add-ons

Not seeing your add-on in the Manage game & add-ons list? Sign in to
My Games on Xbox.com with the Microsoft account you used to purchase the DLC to see what add-ons are available. If it’s not on that list, it’s not associated with that account.

If you bought in-game currency such as V-Bucks or FIFA points and you’re not seeing them in the game, confirm your completed purchase by signing in to your
Order history.

If you see your in-game currency in your purchase history, try quitting and relaunching the game:

1. Press the Xbox button
    to open the guide.
2. Focus on the game, press the Menu button
   , and then choose Quit.
3. Relaunch the game and check your in-game wallet.

If it’s been 24 hours since you’ve done the above and you’re still not seeing your in-game currency:

• For games published by Microsoft, contact
  Microsoft support.
• For other games, contact the game publisher. Here’s
  
  a list of major publishers and their websites.

Hi here is the guide from the Xbox Support website about troubleshooting Add-ons.

https://beta.support.xbox.com/help/games-apps/my-games-apps/view-downloadable-content

Troubleshooting add-ons

Not seeing your add-on in the Manage game & add-ons list? Sign in to
My Games on Xbox.com with the Microsoft account you used to purchase the DLC to see what add-ons are available. If it’s not on that list, it’s not associated with that account.

If you bought in-game currency such as V-Bucks or FIFA points and you’re not seeing them in the game, confirm your completed purchase by signing in to your
Order history.

If you see your in-game currency in your purchase history, try quitting and relaunching the game:

1. Press the Xbox button
    to open the guide.
2. Focus on the game, press the Menu button
   , and then choose Quit.
3. Relaunch the game and check your in-game wallet.

If it’s been 24 hours since you’ve done the above and you’re still not seeing your in-game currency:

• For games published by Microsoft, contact
  Microsoft support.
• For other games, contact the game publisher. Here’s
  
  a list of major publishers and their websites.

I am currently trying to get in contact with Activision support. They are hard to get ahold of.

Sorry for the inconvenience. For whatever reason this game either for Xbox One or 360 is no longer available for purchase from our Xbox store. The game is still available to play but must be purchased from a third party store.

No, it's not true. Once a game is delisted from the store, it can no longer be purchased, unless the game is also available on disc. In the case of Snoopy Flying Ace, that game can no longer be purchased, as it was removed from the store as you know, and was not released on disc.

Install it from your library, not the store. Even if something is no longer available for purchase it, the purchase is tied to your account and you can reinstall it.