Why do you want to delete an item from Studio? In my opinion it’s ok to just have the Successful and Failed status from Studio because otherwise you risk to have two Robots for example that set the Deleted status on the same queue.
Now, the workaround would be via http request or API. But the question now is how do you retrieve the items from Studio with status Successful or Failed? Get Transaction Item retrieves an item with status New. So if this cannot be achieved through a POST request, I’m thinking you can get those items by Id and Select which property to include in the request.
From what I know the DB can hold up to 10 million items. I will have to confirm this with QA but I remember that items that are processed older than 30 days were automatically archived and moved to ElasticSearch. But if you really want to get rid of the item I think you can write a script to delete the items with status Successful and older than 60 days for example.