Plenty of agencies keep a 'scope changes' tab. It works right up until the week gets busy and nobody updates it — which is exactly when scope creep is happening.
A spreadsheet is a fine place to recordscope decisions once you've already made them. It's a poor place to discoverthem, because that requires someone to re-read every client email against the contract, every time. That's the part MarginFlow does automatically — see the mechanics on the how it works page.