Hurd predates Linux by about a year, but was under stop/start development for several years before that too. https://en.wikipedia.org/wiki/GNU_Hurd It's as old as dirt, in computing terms. It was intended to be the final part of a fully GNU based operating system, everything else having been created by that stage.

Stallman et. al. have promised since the late 80s that this would be the future, and at various stages promised that it will be ready for production work within the next year (or two). Like any promises made by Elon Musk, everyone in the tech industry has long since learned to ignore them. Maybe some day it'll be done, but I'm highly skeptical it has any chance of building up the momentum it needs.