In my opinion, KB3156421 is a Cumulative Update which may update almost every system files in Windows 10. Having said that, I suspect that if one system file to be updated by KB3156421 is removed by WinReducer, we cannot install KB3156421 at all.
Sigh, that is one disadvantage of having such cumulative update for Windows 10, as compared to individual updates for Windows 8.1 and earlier. It really prevents us from using the awesome WinReducer, to slim Windows ISO.
I am hoping for winterstorm2050 and other experts to solve this matter.