The information won’t necessarily be lost, because most/all? the instances that were previously federated with the shut-down instance should have cached copies of its activity. Not sure on what scale, or how far back, content caching will be, but I imagine admins have (or should have) the ability to configure that sort of thing

Now, from the perspective of a user looking to migrate to a new platform, not being able to “take it with you” is a valid concern. Mastodon seems to have account export nailed down, but lemmy/kbin are still pretty new and might need time to implement something like this