Google will automatically delete all of a user’s Android backup files if the user does not use his phone for two weeks. The auto-delete function was discovered this week by a Reddit user who used it to create backups for a defective Nexus 6P. Google backups include very sensitive data such as Apps, Call History, Device Settings, Contacts, Calendar, SMS (Pixel phones only) and Photos & videos. Storing backups offline or using specialized backup & restore Android apps is an alternative.
Source: https://www.bleepingcomputer.com/news/mobile/google-will-auto-delete-android-backups-if-users-dont-use-their-phones-for-2-weeks/