When you delete a post you might do it by accident. You can then undelete it, which requires the post still exists (a soft delete).

It appears Lemmy clears the content after 30 days to make it a hard delete (by federating an edit action to change the content to Deleted): github.com/LemmyNet/lemmy/pull/3208

If you want to wipe right away, you can edit the post then delete it.