Why must Listen Server support go
I don't understand the reasons for removing Listen Server support. Odamex and Zandronum (not sure about ZDaemon) don't even support it. Why are you removing this convinient feature, especially when one only wants to play with 3 other people; an instance where a dedicated server would be rediculous.
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If you mean the ability for the client to join games on the local network then this feature has not gone anywhere; open the taskbar, click the Doomsday Engine logo to reveal the application menu and then choose 'Multiplayer Games...' to open the multiplayer game browser UI - here you should find any games currently running on your local network that Doomsday has found through UDP.
If you mean the ability to launch a new server from within Doomsday through the UI, then yes that functionality is not currently implemented. It will return, however. For now one can use the Doomsday Shell tool for this.
In practice this means that the ability of the client to set up an impromptu network game where the client itself acts as the server is no longer possible. However, this functionality will be replaced with a built-in UI for launching a new Doomsday Server instance (which, you can then administer using either RCON from the Doomsday console, or, using the Shell). We haven't yet gotten around to implementing this UI, however, the Shell is very easy to use and so its not that urgent (IMO). Give it a try