Monochrome vector icons make things like dark mode easier to implement. Such small software shops like Microsoft unfortunately cannot afford to hire designers to do multiple sets of icons.
I had to chuckle. I remember back in the Windows 3.11 days, there used be a separate monochrome set of icons for all Windows (and Microsoft) programs.
Unfortunately, we’ll probably be at Windows 20 before the last of the Windows 8-style monochrome icons finally finds its way out of Windows, because that’s how things go at Microsoft. (It’s harder than it used to be, but I’m pretty sure you can still find a few Windows 95-era icons hanging around, and there are definitely still some 2000 icons in there).
Also, many of the DOS applications with icons in moricons.dll probably still run in 32-bit Windows 10. NTVDM and DOS application support don't exist in 64-bit Windows, but they're still alive and well in 32-bit Windows (albeit an optional install, because MS would really rather people not still be using DOS applications in 2021).