Only the main OS image is immutable, but there's distrobox, homebrew, virtual machines (now moved to bazzite-dx, only a rebase away), lots of ways to install stuff in userspace.

Distroboxes are actually a really good developer workflow (except for system level stuff) once you adapt, install everything needed for a project, develop, archive the environment, all without touching the main OS. Obscure old project with instructions only for Arch, needing old python and CUDA versions, no problem. Super versatile and you never boot to a broken system.

In a pinch you can also change the image with rpm-ostree install.