Nix / NixOS

It must be nice to have never productionized a prototype; meanwhile, in the land of startups, putting experiments into production is standard operating procedure, despite being a bad idea.

You're making assumptions. I worked in startups that did took great care not to beta software or libs. We were in production and changing stuff all the time - multiple times a day, but that was one of the rules. When you work with time-sensitive, always on environments with world-wide customers, bringing down production for multiple hours could mean the end your company in those early stages.
Maybe I was extra-ordinarily lucky with the startups I was in.

Just because other startups do it, doesn't mean that it has to be done.