I like SerenityOS' "standard library". It has some Rust-like features (like ErrorOr, which is a lot like Result) but also some C++ usability enhancement that make use of modern C++ features rather than trying to force a C++-shaped peg into a POSIX-shaped hole. The only exception I know of is the pledge() implementation (which uses a space separated string rather than something like an array of enum values), but having pledge() at all is a pretty major improvement.
SerenityOS' original "make no use of external libraries" approach made one of the more complete re-imaginings of the C++ application landscape I've seen. It really changed my perspective on what C++ could be.
I was curious, and a bit distractible this morning, so I took a look at their git repo. I did some osdev tinkering in college so I find this all interesting. AK seems to be their standard library that you were mentioning, but there's some interesting things going on in their Libraries dir too.
SerenityOS' original "make no use of external libraries" approach made one of the more complete re-imaginings of the C++ application landscape I've seen. It really changed my perspective on what C++ could be.