The problem with soft-deleting users right now is that Flarum expects all users to have a valid email address.
If you delete a user for GDPR reasons, then most likely you want to remove that email. Just allowing null would probably cause multiple issues with what Flarum expects at the moment. If you also want to remove the username, then there's no longer any way to visually link posts to a deleted user anyway.
I'm not sure how we want to handle that, and it's probably not a priority right now given there are other, more manual solutions.
An extension could easily create some kind of users with deleted private info.
Regarding the mass deletion of posts, I think it's also best left to an extension for now.