And this UI looks way, way worse and doesn't fit the pixely style of Minecraft at all.

That look reminds me so much of Steam's original UI from the time when it was very Windows 95-inspired, too, but used different color palettes, especially a lot more greens. There's not much of Steam left that will show up with that original UI style (but it still feels like there's some lesser known dialogs that still can show up in that style), so these days it is better known as the "Counter-Strike 1 UI".

https://cs16.samke.me/