Right now, it is not easily configurable via SiteSetting.
Behavior is controller by NarrowDesktop
The potential solution would be to create a plugin which would overwrite isNarrowDesktopView function to always return true.
Something like initializers/always-narrow-view.js
import NarrowDesktop from "discourse/lib/narrow-desktop";
export default {
name: "always-narrow-view",
initialize(container) {
NarrowDesktop.isNarrowDesktopView = () => true;
}
}