It literally takes 10 seconds to come up with a strategy that can detect whether internal preview assets are in use. They don't bother because they are lazy.

game_mode=alpha placeholder assets are allowed

game_mode=beta placeholder assets are allowed, warning issues during build

game_mode=production build fails for any remaining placeholder assets

Even enterprise fucking Java has plugins that can accomplish the above with cascading resource lookups and support for build plugins, I don't see how a game engine or custom build could not do such a simple task.