For those looking for active alternatives or forks, Blades CSS (https://blades.dev/) launched earlier this year as a fully compatible, actively maintained successor, and is already adopted by the Flask/Jinja community website, Grav CMS v2, and FastHTML.
But it's not a drop-in replacement. (I fixed several annoyances I had while using PicoCSS.)
https://github.com/anyblades/pico — underlying fork