Btrfs snapshots are already used in openSUSE microOS which is branded as immutable. And AshOS generalizes it for any kind of distro: github.com/ashos/ashos . I think it is nice middle-ground for regular distros, which does them more reliable.
But for me, immutables are more about separation between the base system and the apps, where the base is not only immutable, but image-based: ostree, A/B partitions, systemd-sysupdate. And the apps are distro-independent: flatpak, containers, and so on. So apps are upgraded independently from the system, and one doesn’t need to upgrade the system just to have apps updated or vice versa. Btrfs snapshots doesn’t solve anything here by itself.