The one mentioned by OP fedoraproject.org/onyx/
Btw, there are so many immutable Linux Distros its crazy. Fedora has four flavors, Ubuntu has one, OpenSUSE has one too. Popular one is https://vanillaos.org/
Even the Steam Deck OS is immutable.
I like tweaking my Linux setup and I have old NVIDIA discrete card, so I don’t want to fight my OS to install a driver or to make changes. But I never had the experience working on Immutable Linux OS so I can’t judge yet.
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