It does say in iOS that your battery needs to be replaced [1]. Everyone keeps saying they never told anyone about this but they basically announced it to the press with a statement (as well as the notice in iOS) when iOS 10.2.1 was released [2].
That's because there is no low-speed mode. It changes based on a variety of conditions including how degraded the battery is (the OS tells you when it needs to be serviced once it's near the end of its life, 500 cycles), cold or hot temperatures, and the remaining battery life (low power mode decreases performance).
How did you come to this conclusion? My 6S was running in this “low speed mode” (as confirmed by Geekbench 4 results) even when the battery was at 100%, phone plugged in, placed at my desk at room temperature all afternoon. Only relevant variable seemed to be the battery age, and the results improved (along with subjective perceived performance increase) only when I recently had it replaced.
I read both, and couldn't find any verbiage indicating they limited the cpu speed based upon the condition of the battery. Only that they "fixed a bug" to reduce "unexpected shutdowns."
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.
Millions of people own iPhone's and don't jump up and down meticulously reading every press release or notification. Hell I can't remember the last notification that didn't catch be at a bad time, prompting me to punt it off my screen as quickly as possible.
I'll run the speed test later to confirm. But there is absolutely NO warning in iOS that I can see that says it's been put into low-speed mode.