When using file on case, give some (permanent) indication that the original has been filed

In the original implementation, some care needed to be taken to avoid mismatches in the case story and the revision history, but for a few releases now the system makes a copy, and marks the original deleted, which for non-case activities makes it completely invisible and unrecoverable anyway, so the suggestion is to simply change the subject to indicate it has been filed.
If you file-on-case an activity that is already a case activity, then the delete makes sense.