You can set per folder: IF you set to 0, none will be retained. It may take some time to apply
https://video2.skills-academy.com/en-us/powershell/module/exchange/set-publicfolder?view=exchange-ps
-RetainDeletedItemsFor
The RetainDeletedItemsFor parameter specifies the retention time for deleted items.
Or for all the public folders:
-DefaultPublicFolderDeletedItemRetention
The DefaultPublicFolderDeletedItemRetention parameter specifies the default value of the length of time to retain deleted items for public folders across the entire organization. This attribute applies to all public folders in the organization that don't have their own RetainDeletedItemsFor attribute set.