As someone on a different Lemmy instance (Lemmy.ca), I think you’re asking to defederate from everything right?

Because out of how Lemmy is designed, if you federate with it, you send it copies of your posts which it stores. That’s by intention. It does respect if the original server tries to delete those comments. But if the original server just disappears one day, those comments will remain copied on the Lemmy instance. In fact, this is designed to stop something like the Reddit thing happening. If any singular site decides to shut-up-shop (like Reddit is doing), the content will be copied elsewhere.