I don't know when we stopped teaching this concept. When I started using the Internet some 30 years ago, there was a broad understanding that once something has been published, it can never be truly deleted. The individual was expected to know this, and avoid publishing information they might want to conceal.

That's not just for Lemmy or the fediverse, but for everything on the internet. When you "delete" a Reddit post, Facebook post, X-cretion, or any other published content, it remains available to anyone who had a copy of it, as well as anyone with access to the service's data. Including any cloud provider they might use, and whatever backups they might make.

True "deletion" is only possible on a machine that you fully control, and only if you deliberately render that content unrecoverable. Anything less is just "hiding" it away. Hiding it away may be "good enough" for practical purposes, but there is a significant difference between true deletion and what a service provider calls deletion.

With Lemmy, the original you upload to your own instance is copied to every other federated instance. There are provisions for your instance to inform other instances that you would like that comment deleted. But, there is no enforcement mechanism. They can't be digitally compelled to remove your content. They can ignore that deletion request; they can even use that deletion request to flag your content for blackmail purposes, if they want to.

Privacy laws like the GDPR can provide a legal obligation to delete your data, much in the same way that a speed limit sign can provide a legal obligation to drive less than 35mph. A legal obligation to delete is obeyed about as much as a legal obligation to not harass people about their car's extended warranty.