> "If you start at a new job and find something resembling a backlog in many areas, for your own sanity, thrown 90% of this out. It’s most likely a good idea to throw anything out that hasn’t been updated in a couple months. In start-up time, after a couple months it’s old news and no longer relevant."

I would take it all and consolidate to a single system so that there is one single source of truth. Do not throw things out, just prioritize them down where appropriate. But if you throw them out, you land in a CS problem where you get dinged on reviews because "I asked them for this months ago, multiple times. My request was ignored and they say they have no record of even having heard me." Of course, you are not going to do every little one-off request that comes in, but if you don't keep track, you don't know if it is really a one-off request vs. the 8th time some little thing has been asked for in the last 6 months.

Your customers do not care about "start-up time". They want a sustainable, stable product where their feedback has value. Don't throw out that feedback.