Not really. Immutability can be overriden by root, who can then edit files.

And in addition to that, /etc/, system config files, including pam files mentioned here, are not immuable even in immutable distros.

I don't know that the current crop of immutable distros would be able to prevent something like this. rpm-ostree, at least, lets you install out-of-tree rpm packages to the base system, you just have to reboot for them to take effect.