Will backup data persist when files/emails or even users have been removed from Google cloud?
File and email data
CubeBackup is a backup service, not a synchronizing tool. All backups, whether they include Google Drive files or folders, Gmail messages, Google Contacts, Google Calendar events, or even entire user accounts, are permanently saved in the backup files and not affected by future changes you may make online. These files can be restored to any state in their version history at any time.
Even when a user is removed from your G Suite domain, CubeBackup will still keep all data for that account as part of your backup. The data for the removed user can easily be restored to another active user.