As you know, you can enable retention policies on SharePoint Online and OneDrive to keep records of your data for a defined period.
Off course, these policies applies to files stored on SharePoint, including PST file.
The retained data is kept on a dedicated list called Preservation Hold Library within the SharePoint/OneDrive site; which means retained data is consuming part of the storage provisioned for the site.
As it is not possible to save only the updated data (which will be every time the file is open) from a PST (like an Office document), this means the versioning of the retained PST file is always the full PST file. This can then lead to have the full allocated storage being consumed.
Starting August 16th, 2021 the number of version retained for PST file is going to be set to 30 days worth of PST version instead of continuously keep the version.
While you should not have anymore PST file in your environment as they are not secured, have quite limitation and easily corrupted, if you still have PST files stored in SharePoint and want to keep the current versioning capability (keep all version according to the retention policy) you will need to opt-out for this change using SharePoint Online PowerShell version 16.0.21411.12000 (published June 25, 2021 – available here https://www.microsoft.com/en-us/download/details.aspx?id=35588) and set the DisableOutlookPSTVersionTrimming parameter to True
If you need to op-out, you must configure it before August 13
Connect-SPOService -Url https://<your tenant>-admin.sharepoint.com
Set-SPOTenant -DisableOutlookPSTVersionTrimming $true