system
A chat on subreddit /r/pcmasterrace
human
The department of veterans affairs entire EHR (electronic health record) is a system that was built in the 70s in a programming language very few people know. But it's consistently won best EHR by physicians even up until recent history. It's highly customizable and because it was built for computers with KB of memory, it's silly fast.
gpt
Some of the UK government's corporation tax software still runs on VAX mainframes. They were keeping actual machines in a basement in Wiltshire for a while I believe, and then a few years ago there was a project to move the software onto emulated boxes, I have no idea if it was ultimately successful.