Just got this notification on the Xbox mobile app:

Due to a new policy starting October 1st, 2023, existing and new captures on the Xbox Network will be automatically deleted after 90 days. The first deletions will start in January 2024. To back up your game captures before they’re gone, select Get Started below. To learn more about how to back up your game captures, select Learn More below.

The link takes you to https://support.xbox.com/en-US/help/games-apps/my-games-apps/back-up-game-clips