It's hard to say. The claims that Apple makes about performance tend to coincide with device launches. That's when they really do talk the most like, “This phone is faster than the old phone; this is roughly by how much.”
Most people in the industry look at the figures that manufacturers present as an optimistic best-case scenario. A device manufacturer will find the one benchmark that shows a 2X performance and will say, “up to two times the performance”.
There are also, of course, factors that are outside a device manufacturer's control that might affect the performance of its phones. A great example of this would be thermals, where if your phone is in a hot environment, it will more likely run slower than the phone in the cold environment, simply due to the mechanics of how modern integrated circuits work.
In this particular case of having the battery start to slow down, I think Apple wasn't as forthcoming as it could have been about the condition of the battery. We've heard reports of users with a slow phone, and this was ahead of Apple's battery upgrade program that it announced in late December. They would know their phone was slow. They would be able to verify this with Geekbench or with other tools, and they would take their phone into an Apple store and say, “My battery is slow; I would like to upgrade this”, and Apple would say say, “No, no, your battery is fine.” Clearly, Apple wasn't being as forthcoming as it could be.
To speak particularly to deceptive claims, I'm not sure how much Apple claims of its iPhones 6s two years after they've been released. The question is, of course, do consumers expect performance to degrade over time? I would expect the average consumer would not. Consumers are used to the idea of battery charge decreasing over time. As I mentioned before, lithium ion is a fairly established technology and everybody understands the batteries will reduce capacity over time. I don't think anybody expects their phone to suddenly get slower because of the quality or the age of its battery.