As discussed in CONTRIB-9360 we found that its intentionally not to delete recordings when a BBB activity is deleted automatically.
After discussion with Fred today we suggest to add a feature in admin settings an activity instance settings that all recordings related to a instance will be deleted witrhthe activity.

Settings:
Delete all recordings with activity:
yes: with the activity all recordings are deleted
no: recordings are not deleted and will be available for course backups or restored courses.

The admin can decide which setting is enabled per default for all new instances
The admin can decide that the setting is used for all existing instances

Session recordings will include personal data from users. If the recording is notdeleted before the instance is deleted it will stay for ever in BBB and can not be deleted anymore This is a critical privacy issue.