Ok, that's just nasty. Not everyone can handle 3.1, Legacy based or otherwise, so you cut them off from their only means to find players? That's literally ethically wrong.
For 2.3.9 players, you can plug your game into warzonelegacy.org for the masterserver part of your config file. Your config file is located either in ~/.warzone2100-2.3/config if on Linux, or My Documents\Warzone 2100 2.3\config on Windows. This is a text editable file. Opening it in Wordpad is preferable if you are a Windows user. This will allow you to use the Warzone 2100 Legacy lobby server. Since Legacy doesn't identify itself different from normal Warzone (besides calling itself 1.0 whatever or microwave whatever) in the lobby, it's going to make things a little confusing
, however, I find it more important to give you people a "refuge". So go ahead and adjust your configurations if you are still on 2.3.x. I won't tell me if you don't. ;^) I have set up my bot (pr0t0) to guard a channel on freenode called ##wz239 (double pound) where you can chat and find players.@Devs: If people are still on 2.3 in such numbers, perhaps rather than trying to force them to use a version they might not even be able to run, offer something they CAN run or let them use the old? For a long time, years, 2.2, yes, 2.2 was supported on this server even though it provided a warning.
3.1.x users: Do NOT plug yourself into warzonelegacy.org! Legacy has code that allows it to communicate properly with the different lobby software, 3.1.0 does not and will crash to a desktop frequently. 2.3.x has primitive enough netcode it is not affected by this problem to my knowledge.
contra principia negantem non est disputandum