On Lemmy any comment you post gets federated out to other servers, so it’s available to anyone who sets up a server. So by design it is not possible to control who gets to see or archive your comments. I could set up a server to permanently archive every comment it sees, and if your server sends me your comment it goes into my archive. Probably people are already doing this for data mining. It’s not clear that you could bolt some kind of privacy control on to this architecture, which is fundamentally designed for sharing.