Complex sentence: With the 6.15 release in 2000, Microsoft discontinued support for MASM as a separate product, instead subsuming it into the Visual Studio toolset, though it was still compatible with Windows 98, though current versions of Visual Studio were not.
Split into simpler sentences: With the 6.15 release in 2000, Microsoft discontinued support for MASM as a separate product, instead subsuming it into the Visual Studio toolset. Though it was still compatible with Windows 98 current versions of Visual Studio were not.