Hi @Ian Maclauchlin,
There has been a long running uservoice open for this here.
https://office365.uservoice.com/forums/273493-office-365-admin/suggestions/9297432-automatically-rem...I would usually say vote for this, but this hasn't been picked up in a significant time as it was opened in 2015! This means its very low down on the priority list and may not be picked up for a long time. I have been asked periodically about it over the years.
Removing the licence does not automatically delete the data. When a user's license is removed, data that is associated with that user account is held for 30 days. After the 30 day grace period, the data is deleted and can't be recovered. Files that are saved in OneDrive or SharePoint Online are not deleted.
See article here
https://docs.microsoft.com/en-us/office365/admin/subscriptions-and-billing/remove-licenses-from-user...Hope that answers your questions!
Best, Chris