I’ve never worked with Unreal’s server setup, but I imagine it doesn’t absolutely require to use their code, right? You can still make an Unreal game on the client and use something else for your server, meaning there must be some sort of common interface between them.

The point is yes, there is going to be code you can’t legally release, libraries you can’t use, but you can release what code you can, and then leave the interfaces for code you can’t, leaving hobbyist devs to pick up the slack. You can even make servers from scratch that way, as with stuff like AzerothCore, where all of the code was figured out from scratch based on packets from client to server and studying hex code for hours. Technically AzerothCore was just building on top of MaNGOS but that was created using packets.

Even if you strip out the code you can’t legally release, that’s a hell of a boost to development that you wouldn’t otherwise get.