Since Windows 10 v1903, icons have been moved from
C:\Windows\SystemResources\some-file.dll.mun (not .mui). See this other SuperUser post and Microsoft's website.
MiTeC's Icon Explorer is a free and convenient tool to search through folders and identify files with icons. It also appears to be smart enough to search
C:\Windows\SystemResources instead of
Note 1: One limitation I found: MiTeC's Icon Explorer shows icon indices but not icon IDs. If you're using the Windows Registry to find/edit links to a specific icon DLL, and the icon number has a negative sign, then the number refers to the ID of the icon, not its index. See here and there for more detail.
gameux.dll.mun doesn't exist in
C:\Windows\SystemResources, maybe because the Games folder was removed in Windows 10 v1803: see this article. A game folder icon still exists in
imageres.dll.mun though (index 177, ID 186).