If you have been following alternative operating systems for a while, there is a good change that you have herd of Hurd. It has been in development since before Linux, and yet it is still not stable. This kernel with a super cool logo is super powerful. I am going to tell you where you can get it, why you might want to check it out, and some of it’s drawbacks.
There are part of a category of “Vaporware” but inside this, there are distinctions: Vaporware that is stuck in development hell, just like Hurd and the 0AD game, and the Vaporware that was promised, its not officially canceled, maybe can return somehow, but its not sure that it even exists anymore, like Half Life 3.
You should check 0 A.D. more currently.
One thing is that devs consider it Alpha, other is being not playable at all.