On the Qt website, in the download section, there is a pre-compiled Qt5.9 available.
Before Qt5.9 there were different pre-compiled packages for each Visual Studio version.
Is new pre-compiled Qt5.9 VS version independent? If not, for which version is it?
No, it's not. It can't be since visual c++ versions are usually* not binary compatible (*just now Visual Studio 2017 is binary compatible with 2015).
That package just has different versions you can install, to target Windows: VS2013 (x64), VS2015(x64 and x86), VS2017(x64), MinGW and to target Android (arm and x86).