However, once a comment had been federated to another instance, I believe that’s up to whatever instance it’s propagated to

Yeah that's a very good point. However, your instance will have fulfilled its responsibility, at least.

At the end of the day though, anything you publish online should be considered lost, in a practical sense. Even if lemmy adds a feature to facilitate permanent deletion, it's up to each instance to actually implement it - much like it would be up to internet scrapers to delete the content they collect from anywhere and everywhere.

lemmy isn't without its bugs - like I mentioned in another comment, it's now possible for users to view deleted comments, at least in certain apps (in Jerboa, if you start a reply to a deleted comment you'll see it). But it's steadily evolving.

I'm still waiting for more instance agnostic links for things. feddit.uk/comment/123456 is different to lemmy.world/comment/123456, but what we should have is something more like a federated host at feddit.uk/comment/123456, then copies should be lemmy.world/comment/123456@feddit.uk. As it is now, there is no straightforward way to go from a comment or post in one instance to the same in another. You have to manually search the community or user. I think maybe there are ways for apps to do this through the back-end API, but this should really be there on the front end. It's annoying when someone gives you a link to another instance and you can't reply to it.