This resource provides information on a retention period after creation with an auto delete option.It seems you are incorrect about it being unsupported by Wasabi.
You can set the compliance policy on any bucket controlling all the objects that are stored in that bucket. Specifying the bucket compliance policy with the following XML tags.
RetentionDays: An integer for the minimum number of days that objects are always retained after their creation date or release from conditional hold. You can extend this retention date for any individual object, but may not shorten the date. This parameter is always required.
DeletAfterRetention: A Boolean value indicating if the object should be deleted automatically at the end of the retention period. The default is to not delete objects after the retention period. Note that this setting may be changed even after the settings are locked.
The compliance settings for a bucket are specified using the “?compliance” query string along with the compliance settings as the XML body in the request. For example:
PUT http://s3.wasabisys.com/my-bucket?complianceHTTP./1.1
<BucketComplianceConfiguration>
<Status>enabled</Status>
<LockTime>off</LockTime>
<RetentionDays>365</RetentionDays>
<DeleteAfterRetention>true</DeleteAfterRetention>
</BucketComplianceConfiguration>
After compliance is enabled for a bucket, the policy is immediately applied to all objects in the bucket.
These are the docs which I found these excerpts from: https://wasabi.com/help/docs/
A step by step guide: https://wasabi.com/wp-content/themes/wasabi/docs/User_Guide/topics/Enabling_Compliance.htm
Hope this helps friend.