I remember when WinAPI apps used development tech that was backwards compatible for Win9X/Me.
That was a long time ago. Have you tried getting something modern to run on 9x kernels? There's so many missing API calls it quickly stops being fun. Anyone who needs backwards compatibility that far back should revise their technology stack.
Only supporting Windows versions down to ~4 years ago is a bit excessive, though.
Windows App SDK (formerly Project Reunion) was designed to enhance native Win32 and .NET 6 applications, and migrate existing UWP apps to Win32 or .NET model by bridging the gap between low-level Win32 APIs and high-level programming concepts in .NET and UWP. There is only so far you can go for backwards compatibility.