Isn’t that what they did? https://www.knockoutcity.com/private-server-edition#section-download

From the article:

There’s one thing, however, that Harrison recommends studios do above all others when sunsetting a live service game: let players keep playing the game on their own servers. Before shutting down Knockout City, Velan released the game as a standalone Windows executable with private server support. It’s still available to download.