First time I hear about Duranium

Immutable postmarketOS (project name: Duranium) is an image-based variant of postmarketOS that provides atomic A/B updates with automatic rollback on boot failure. The system uses a read-only /usr partition verified by dm-verity, ensuring the operating system files remain unmodified.

https://wiki.postmarketos.org/wiki/Duranium_(Immutable_postmarketOS)