There is also Gobolinux, that do so in a less stringent fashion.

This in that, as i understand it, compiling a lib with a feature enabled or disabled will produce different tree branches in NixOS/Guix, while Gobolinux will happily replace one compile with another if it is of the same lib version number.

So you can't have lib foo 1.0 with feature X enabled in one compile sitting side by side with the same foo 1.0 with feature X disabled when using Gobolinux.

Then again, Gobolinux has never been meant as a server distro...

Edit: thinking about it, Gobolinux could perhaps be extended with checksum dirs inside the version-subdir to separate different compiles. Not sure if this has been considered or attempted.