Backup history of the Teams service on the Preview version will be deleted when this preview ends in November, as we migrate backup sessions to a new database.
Backup and Recovery of Microsoft 365's Teams service is currently free while the service is in it's preview phase.
The preview phase will end in November, and at this time Teams will transition to a paid service, billable by the number of unique owners of Teams.
For full details on your billing, please contact your account manager.