And a book about spam filtering could be reduced to the sentence, "Just make sure users only get the email they actually want." Since the filtering of both email and personal possessions is rather more complicated than that, we end up with entire books written on the subject.

For email you might use a combination of DKIM+DMARC, whitelisting, blacklisting, graylisting, string matching, and bayesian filtering based on word corpus. I guess nowadays you might be using more sophisticated ML techniques but I'm not really familiar with email.

For personal possessions, Kondo recommends sorting items by category before discarding things, she has a bunch of tips on how to store things (don't get a bunch of boxes, don't store stuff in a pile so you never see whatever is at the bottom), and a bunch of tips for how to sort through each category of item.

Common sense applies… I heard somewhere in this thread that someone throws anything out they haven't used in six months, but if I did that, I'd have to buy a winter jacket every year (and an air conditioner, camping gear, etc.). That doesn't mean the rule is bad, it just means it wasn't written like a computer program.