As far as I know, Lemmy doesn’t allow the deletion of content. https://www.reddit.com/r/privacy/comments/1b6g219/psa_you_cant_delete_photos_uploaded_to_lemmy_so/
You should however be able to overwrite your posts. So when you delete them, change every field to random nonsense.
I occasionally use it. You can message me at tsugu@disroot.org if you want.