I know that some ERA communities created their own Discord servers. Is there a list of them all, or are they secret in nature? Would love to join some, but I'm not even aware which topics/games/issues spawned a Discord branch.
They are tied to communities. Find the communities you like in the hangouts section and the discord for that community (if they have one) should be listed in the OP.
For example, PC Era has a pretty active one. That thread isn't in hangouts either. Some game OTs might even have one (it might link to a discord for the entire series like the Xenoblade one).
This question has been answered, and the thread is now closed. The best way to find related discord servers is to go to community threads of things you like in hangouts and ask if there is one.