I experienced a case of zero one many that would have been better if YAGNI was applied. We were building a messaging system for a system that was being rewritt

I experienced a case of zero one many that would have been better if YAGNI was applied. We were building a messaging system for a system that was being rewritten, where the only requirement was direct messaging between 2 users. It was suggested to generalize it to support multi-user conversations since that was a product desire some time ago. This complicated the backend implementation significantly, including performance and maintainability. The APIs and the frontend only supported 2 user messaging still. And the system never needed multiuser chat.