"vs16" stands for the Visual Studio version with which the software was compiled.
Originally from a source that is not allowed to name here (...you know 😉...) but via duckduckgoing 100% verified:
More recent versions of PHP are built with VC15 or VS16 (Visual Studio 2017 or 2019 compiler respectively) and include improvements in performance and stability. (Source)
- Michael Sy.