Earlier quoted context omitted.
It's a Steam issue, given that background services don't have to run as SYSTEM, and yet Valve decided to have Steam's background service do precisely that. Thankfully, the Linux version doesn't seem to have this problem (AFAICT).
I think it should become a Windows/Microsoft issue, to be honest. A good example is the recent Zoom vulnerability, the software made it possible to perform certain exploits on the user and operating system, so Apple stepped in and disabled the exploit. In this case, I think Microsoft should do the same in order to protect their users. My guess would be that the install base of Steam on Windows is as least on par with…
Windows does support this functionality, and ultimately Valve's to blame for not using it, but you're right that Microsoft should be more proactive in encouraging good design and discouraging bad design.