Nor they have mentioned Puppy Linux. It uses SquashFS images and layer them on each other. You can choose to save your modifications in a new image, or discard

Nor they have mentioned Puppy Linux. It uses SquashFS images and layer them on each other. You can choose to save your modifications in a new image, or discard them. E.g. you can run a Puppy Linux from a CD-R (one time writeable), by appending all your changes. I think that's a great model to be immutable, but AFAIK Puppy Linux doesn't have the convenient tools to manage these snapshots, switch between them, roll back and such, and they don't seem to go in that direction. (I used Puppy Linux as my default system for a while, but I lost touch with them and I don't know how are they doing now.)