Managed instance backup storage billing went into effect November 1, 2019. Backup storage is free of charge up to sizes equal to the size of the provisioned storage, while the use of any additional storage capacity is charged at a fixed amount depending on region (see pricing details).
For example—If you provision 1 TB of storage, you have the same amount (1 TB) of backup storage for free while any additional use of backup storage over 1 TB will be charged.