The simple answer is use a TLD that a library is legally required to backup.

E.g the British Library is legally required to backup all .uk TLDs once a year.

In case of a future disaster/war, mirror your content on multiple top level domains that have similar rules, and your site should hopefully sit in national libraries around the planet for many years.

(Of course, this only really works for static content - server side code can only be backed up if you publish it)