Debian based immutable OSes

As others have said, there's Vanilla and Endless, but both use GNOME.

Is there a reason why it has to be Debian? With an immutable distro, you won't be using traditional package managers anyway (like apt), all have the same stability factor, and with the same systemd + KDE, it's more or less the same experience regardless of which distro you go for.

One of the key usage patterns of an immutable distro is using a container to install your extra packages. So you could go for a Fedora-based immutable KDE distro like Kinoite, and set up a Debian container where you can use apt and other Debian tools.