what does immutable mean?
Strictly speaking, 'immutable' means unchanging. For Linux distros, this means that (at least some part of) the OS is read-only.
On any distro, you could invoke the chattr +i path/to/file_or_directory
command to make a file or directory of your choosing immutable. Thus preventing you or anyone else from changing that until it's revoked.
The so-called 'immutable' distros employ this at the OS-level. However, their implementations (and the implications thereof) may vary significantly amongst them, unless they share some 'heritage'.
Going over the many different implementations and their implications is out of scope for what this comment intends. Especially as the 'immutable Linux landscape' is fast moving. Thus, potentially making it outdated the very next landscape-defining change.
I was intending to, but it got very unwieldy real fast. I did provide some very basic pointers, but nothing earth-shattering. I suppose this is a decent read with the acknowledgement that the author has primarily read up on Fedora Atomic (and not the other 'immutable distros). Which ain't bad for our use as Bazzite is derived from Fedora Atomic anyways.