Everything you post on lemmy is intended to be public, so there really isn't a point to it having privacy features like encryption. There is the one sticking point of dms being readable by the relevant instance admins, perhaps that will change at some point, but for now lemmy supports attaching a matrix handle to your account for secure messaging.

That said, something the underlying protocol (ActivityPub) does have, are standards for both editing and deleting your own posts. It will be quite possible to have everything or anything you've put out there changed or removed, much like on reddit. The action will propagate to all federated instances.

Also I'm not sure what you mean by there being no history deletion in matrix? I run a node, and purging a room, or an entire account, is absolutely possible. You can even set a node up to only retain data for a certain amount of time. Or are you referring to the fact that you can't make another node that has the history for people you've talked to on it, delete it?

There ActivityPub has matrix beat, if you delete your comment, any instances it was visible on (unless they defederated since) will also delete it.