Of libraries that are better than the STL? Or data structures that the STL is missing? Without getting into the weeds of esoteric data structures and algorithms I think there's a few holes that aren't addressed:

https://github.com/martinus/unordered_dense provides better replacements for unordered_map/set.

The STL is missing B-trees and B-heaps, as well as d-heaps.

STL is also missing a radix sort, which is even more sorely missed now that we have std::executor::par_unseq to play with.