Thank you @yepev for the link.
I also found https://github.com/v2v3v4/BigWorld-Engine-2.0.1, which is really old (2012, when WG acquired the engine) and likely not the same at all today. But the protocol core principle could be the same today (edit: fully obsolete). I will try to play with the login process later to guess how much the protocol have changed, I'll also compare it with the disassembled '.exe'.
EDIT1: I first thought these sources were obsolete, but I can actually find some structure looking like "LogOnParams" or "ServerConnection" (src/lib/connection) from my disassembly of WoT 1.14.