98.css – A design system for building faithful recreations of old UIs

I'd claim that this is way better than what we've got after years of UX/UI "research" and "improvements".

And it's not that I'm some old boomer. In '98 I was one year old. I just hate buttons which don't look like buttons, tabs which don't look like tabs, text fields that you have to click to discover they're editable etc.

Have much UX/UI research been put into operation systems UIs since Windows 95?

I suppose Apple must have done some research for MacOSX, but in the past two decades it feels like features are just be thrown in, especially on mobile, with no concept of discoverability or consistency. More than anything I think companies desire to have "consistent branding" or "unique look" is to blame for much of this.

That said I'm not a fan of the black text over dark gray background.

Also, that website includes "show code" buttons disguised as ordinary text.

Yes, Apple at one time had a phenomenal Human Interface Guide for native OS X applications which was based on actual UX testing and research. Things like, "make your application features discoverable," "clearly delineate functional areas and controls," and "use verb+noun on action buttons." It was Good Stuff and it heavily informed a lot of UX decisions for the prominent Linux desktops at the time.

It's still around but I flipped through it recently and it seems to be a mix of the absurdly general ("don't build an app for a specific screen size") to the very specific (MacOS and iOS APIs). Maybe there's some gold buried in there still, but I'm not sure where to find it.