Is there a pure CSS or CSS + JS alternative that does not rely on React?

I wish. React + Tailwind as the commonly accepted default is too bad, it sacrifices portability for convenience. It's legacy code in the making, like jQuery and Bootstrap.

Haven't use it myself, but I've bookmarked https://basecoatui.com/ to try in future.

First thing I thought. Tying it to React is a shame.

Yeah. Neobrutalism is mostly about the drop shadows + bright color palette. You don't really need a component library for what should be some basic CSS rules.