Yes I think it's safe to assume that Apple is not going to be notifying users about CPU speed adjustments and when those occur, given that it changes based on anything from cold to hot temperatures.
Or even a one time notification on OS upgrade. Perhaps with a note next to the "you should replace your battery" text in Settings.app. E.g. "this may affect phone performance, tap here for more info..."
A one time notification would be all they need. I'm an iOS developer and didn't even know that settings tells me when I need a new battery. Once told, I'd never forget.