This is a walkthrough to the Godot codebase over time. I can't publish full repositories, but I will put here the interfaces of some APIs including the year they were created and how they looked like at the time.
This was the first version of the engine as such. It was later on used to create Regnum Oline. This is source code where I am sure I can still put legally here. Here are some class interfaces.
Mesh class: