NixOS then lives in the future, as that is exactly what it does!

Pff, FreeBSD and Illumos/Solaris have BE's (boot environments) since years

Can you give your system’s declarative configuration to another machine and boot into an equivalent one? :D

Yes.

With FreeBSD I can 'send' entire BE to other system and boot it there - which I often do.

Just use:

    # zfs send <ZFS-BE-DATASET@snap> | ssh <IP> zfs recv <ZFS-BE-DATASET>

I also sometimes installed FreeBSD/GhostBSD system in a VM and then 'send' it to physical hardware.

Hope that helps.