Slackware is what Linux used to be.
Cause it didn’t really change at all since the beginning.
Still the same structure under the hood, still comes preinstalled with twm (the original default window manager for X, developed in 1987), if you want it to look like it’s 30 years old.
The distro is even still maintained by the same guy who created it.

But contrary to Haiku it works as a production system in the modern world, too. It has KDE, Wayland and a modern Linux kernel, and you can install optional helper tools that manage dependencies, Flatpak support, Steam…