Your users already know what's relevant. Are you listening?
TL;DR If you work on search relevance, you know the feeling. You ship a synonym. You boost a field. You add a vector model. You stare at a judgment set that was labeled six months ago and hope the next NDCG number moves in the right direction. Somewhere between offline metrics and production traffic, a quiet gap opens: you optimized for what you think users want, not for what they actually do when the results appear. That gap is not a failure of effort. It is a missing feedback loop.