
Hi @Michael Groombridge,
Thank you for posting in this community.
The retention policy for SharePoint Online is for the retention of saved data and does not include retention of deleted data.By default, SharePoint Online retains deleted data in the Recycle Bin (First-Stage) for another 30 days. If deleted data remains in the Recycle Bin (First-Stage) for more than 30 days or is deleted in the Recycle Bin (First-Stage), then the data is moved to the Recycle Bin (Second-Stage) and remains there until the 93rd day of deletion.
Reference:
SharePoint Online data deletion in Microsoft 365
How retention works for SharePoint and OneDrive
For undeleted data, if you want to keep it forever, just set the retention policy to "Retain items forever".
If you don't know how to set up a retention policy, please refer to this article: Create and configure a retention policy.
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