Every product accumulates features that did not pay back the attention they take. Killing them is the right move. Forgetting why you killed them is the wrong one, because the same impulse that built them the first time will build them again in two quarters, when someone new joins and pitches them.
What to record
For every feature you kill: what it was supposed to do, the evidence that it did not, and the reasoning that led to the kill. Two or three paragraphs, dated, in the same place as the rest of your product decisions. That is the entire system.
Why it pays off
The next time the same idea surfaces (and it will), you have a real record of why it did not work. The conversation moves from "let's try X" to "we tried X for these reasons; here is what we learned; if you think it is different now, here is what would have to be true." Product judgement compounds when killed features are remembered.
What this is not
It is not a sacred document and it does not prevent revisiting decisions. It is a forcing function for honesty: if the case for trying again is strong, fine, but you have to engage with what you already learned.