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.
I do know why the backwards compatibility is so shallow, I just find it amusing. At one time MS was all about a very deep backwards compatibility model as I vaguely remember.
Desktop WinAPI code written for Win9x/Me is for the most part still compileable for Win10/11, with some minor tweaks.
There are some aspects of older WinAPI code that definitely hasn't aged well, expecting hardware able to run with a 256-bit display palette for example.
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.