Actually, the c++ lib has never been available to my recollection (only the base CRT). And they don't even ship the CRT with the compiler any more.
Just in case you/any other reader didn't know: That's just because they don't consider the ucrt compiler-version-specific anymore, it's distribution (binaries and source) moved into the windows SDK, E.g. Windows Kits\10\Source\10.0.17763.0\ucrt. The vcruntime is still with MSVC.
But this source never included the STL.
I'll concede I'm probably missing some obvious gotcha.
You mean things like vector, map, etc. ? The source for those is already in the header files.
Now, being open source enables much more.
An example would be if any performance enhancements can be ported to other C++ implementations.