In most blockchains, each computer acting as a full participant in the network holds a complete copy of all transactions that have ever happened since the launch of the network. This means that even under catastrophic duress, as long as a fraction of the network computers remains functional, a complete backup will exist.
In PoS chains, there tends to be far fewer full participants so the number of backups and distribution is far less. So far, this reduced level of redundancy has not been an issue.
