Thanks! Exactly what I was looking for!Steam:
https://partner.steamgames.com/doc/home
Discord:
https://discordapp.com/developers/docs/intro
Humble Bundle:
https://support.humblebundle.com/hc/en-us/sections/200515154-Developer-FAQ
Itch:
https://itch.io/docs/general/about
Gog documentation is private
Epic documentation is private and possibly non existing. Developers do not have access to do anything anyways.
Although they are somewhat sparser than the steam works stuff.