The oldest server binary I've used recently was for Freelancer, which was released in 2003. It required some elevated permissions but otherwise, both the server and the game itself ran fine on newer hardware and versions of Windows.

Failing that, there's always hardware emulation but to be honest, unless you're for whatever odd reason trying to run your server on AMD64, I don't think there's too much to be concerned about. And, if source code is released by the developer, the community that remains could see to continuing support.

Worth noting here that the self-hosting mod for Titanfall 2, called Northstar, already did all of that with nothing but reverse-engineering involved because Respawns servers were basically unusable for years.