> 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 I just had to change a couple of options in my configuration which had become outdated.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.

I'd say not much: you update the channel, run nixos-rebuild switch, fix all the warnings/errors due to renamed/changed options until it succeeds and you're done. If you have a database like postgres you may have to do a schema upgrade manually, since the default version is updated every 4/5 releases or so.

It's very rare to find something that prevents you from directly updating. Nixpkgs tries very hard to no require new Nix features, so it evaluates with even Nix versions from a decade ago. Also, NixOS options and packages are frequently changed, but the automatic migrations (mkChangedOptionModule, mkRenamedOptionModule, alias, etc.) are never removed in practice.

Since the binary cache has never been cleared since its creation (2002?), it should actually be easy to install a super old NixOS release and upgrading it to the latest to see what happens.

By the way, there are LTS versions of NixOS, just not officially supported. See https://docs.ctrl-os.com/.