An immutable Linux distribution is an operating system (OS) that is read-only at its core. That means you can’t easily modify the OS. This includes the file system, directories, applications, and even configurations. Even as an administrator, you can’t make any modifications to the distribution.
If something gets changed in an immutable distro, it’s only temporary and reverts when you reboot. That’s why these OSes are called “immutable.”
This is one of those “because you can” type perspectives. This is not the same as using a Linux LiveCD boot, as those generally don’t keep settings and changes between sessions.
I can liken this closest to using containers, where your OS (or app) boots from the read-only image, but you have volumes which store the user settings changes and documents etc between sessions. A lot depends though on the particular Linux distro as whether it allows the saving of any config files at all, or just limited to maybe the /etc folder.
Obviously, these distros do increase both security and stability (from dependency updates) as the key OS reverts after rebooting.
But, as the article points out, these types of distros may not be for everyone as they have quirks of their own. It is interesting though to see that the Steam Deck actually uses an immutable Linux distro.