If an instance goes down, then so do all the communities on it. Perhaps it the future, you could be able to migrate a community or an account to another instance. I know Mastodon has a future for migrating an account.

Migrating profiles should be possible at some point in the future, as well and importing and exporting content profiles. Not necessarily comments or posts, but your follows and subscriptions.

Regarding communities themselves, posts and comments, that’s much trickier but it’s feasible that the Lemmy community will develop a way to:

A) Archive content in such a way that it’s read only and requires super low resources to host by interested internet archivists. B) but which is still connected to the fediverse so that the original content authors, after authenticating somehow, can request deletion of specific content

Ideally we’d want to associate each Lemmy account to a unique random public key and sign each post or comment with a secret key that only you know. Maybe your instance can hold the key in encrypted form and deliver it to your browser to decrypt with a simple password so that the user doesn’t have to care about any of this. To them it would still be username and password.

Anyways, this would allow you to still authenticate yourself in the far future when your instance no longer exists and request deletion of whatever content you wrote in the past.