Another really nice Immutable Linux system that I'm using is VyOS.. It's targeted primarily at a router OS, but you can run containers on it now to make it pretty versatile.
Same thing for OpenWrt I think. I believe it works by using a squashfs and tmpfs and using overlayfs to overlay the tmpfs on top of the r/o filesystem. But I'm not sure that fits the definition used here for immutable OS:
> We could say that a Linux LIVE-CD is immutable, because every time you boot it, you get the exact same programs running, and you can't change anything as the disk media is read only. But while the LIVE-CD is running, you can make changes to it, you can create files and directories, install packages, it's not stuck in an immutable state.
Basically, it's an image based OS that configures everything from a single config file on boot. https://docs.vyos.io/en/latest/introducing/about.html