> a rolling release like NixOS is exactly the opposite of an LTS distro

NixOS is not rolling release. This is a common misconception. You can use the unstable channel, which is a rolling release, or the regular channels which get released twice a year. These are really stable and move very slowly. You can also mix and match, running software from different channels.

> I actually wonder what would happen to a NixOS installation frozen in time for 5 years that then you want to update to latest all of a sudden

I have done this recently as I kept an airgapped machine, which I decommissioned, connected to the Internet and updated to the latest channel. Everything worked just fine. I just had to change a couple of options in my configuration which had become outdated. Nix is functional, so it's much less prone to all stateful issues that plague other package managers.