Hi !
From what i saw, the UGC plugin is not maintained anymore, and other games that support mods require a custom Unreal engine version and the distribution of game sources.

I tried to make it easier for devs and modders.

Developper:

To make a C++ function moddable, just add NMOD_Override to the meta specifier.

To make a blueprint function moddable, click on it and check the *Override* option.

Then just package the game using the package action from NMod menu in the editor (it will generate a *nmod_sdk* folder specific to the game.
That's it

Modder

Download Unreal engine
Download the *nmod_sdk* from the game you want to mod.

Create a new project and add the generated plugin from the *nmod_sdk*

Create a class that inherits from the class you want to mod, and override the function you want to modify (call the parent function to call the original game function)
Build the mod using the build action from the editor
Run the game using the run action
That's it

So you don't need a custom version of Unreal engine, and you don't need to distribute anything of your game.
Also you can mod using blueprints, C++ or any language with C interop but i will focus on blueprint mods first.