If you don't like modern UIs, I'd recommend Linux today. The main Ubuntu has all the crazy UI changes that I stopped following long time ago, but all the old stuff is still in repos.
Almost 20 years later, the "blackbox" package is still around: https://packages.ubuntu.com/bionic/blackbox . You can install it, choose it in the dropdown from the login screen and start using it. Or get a version of Ubuntu with this stuff pre-configured (like xubuntu or lubuntu)
And the best part? You have all the modern software -- browsers, editors, compilers. You can be fully productive and enjoy the stable UI
For example, this is blackbox 0.60.0 window manager, released around 2000: https://www.linux.org.ru/gallery/screenshots/19788
Almost 20 years later, the "blackbox" package is still around: https://packages.ubuntu.com/bionic/blackbox . You can install it, choose it in the dropdown from the login screen and start using it. Or get a version of Ubuntu with this stuff pre-configured (like xubuntu or lubuntu)
And the best part? You have all the modern software -- browsers, editors, compilers. You can be fully productive and enjoy the stable UI