Making a moddable (extensible) game or game engine

An example of hard-coded (not easily-moddable) and "data-driven" (easily-moddable) programs:

# Not easily modddable:
def program():
    print("Hello world!")
 
# Easily moddable:
def program():
    message_to_print = load_message_to_print()
    print(message_to_print)
 
def load_message_to_print():
    with open("config.json", "r") as infile:
    data = json.load(infile)
    return data['message_to_print']

Wikipedia

Wikipedia - Game Engine

StackExchange

I want to make a moddable game. How does this affect my programming language choice?
- There's good info here.

OFP / Arma

Minecraft

Wikipedia: Minecraft mods - History
2011 - Searge's MCP Mod System
2015.07 - Minecraft forum - The History of Modded Minecraft
2016.06.03 - PacktPub - A Brief History of Minecraft Modding
- So basically, one way to have your game moddable is to have people be able to modify the source code. So it's sort-of open-source in that people can hook into things, but it's not open-source in the sense that people can do whatever they want with your code (like fork it or straight-up copy it).
- This is a really good article.