It looks like Microsoft is adding the official support for game mods to the Store app. A new folder in the latest Fast Ring indicates that Xbox / Game Pass PC client is receiving this feature.
A new folder, Program Files\ModifableWindowsApps, can be found in Windows 10 build 19841. It already contains some resources related to themes, icons, and app logos.
The folder is intended to store resources where users can modify instead of directly changing files of the installed application. This will allow to install mods. Each app that supports mods will have one or several subfolders under the ModifableWindowsApps folder.
It is not clear yet will this feature available to Win32 games and apps redistributed via the Store, or it will remain exclusive to UWP/Store apps. The official announcement could shed some light on this topic.
Thanks to Andrew Bennett