It seems most people more or less feel it's not about what they did, but how they did it, and I agree. There's nothing wrong with optimizing their OS to make more efficient use of the CPU, and in fact that's a good thing. Had they been upfront about it from the beginning, and included it as a separate mode, so the user could choose between optimization and full performance, then it could have been an additional selling point, and the benchmarks could have tested both modes to see what the effects on performance and battery life are. Instead, it just came across as being sneaky, like selling a Ferrari with half the cylinders turned off unless you put the pedal all the way down.
I actually agree with OnePlus, that current phone SoCs are more than sufficient for most tasks/users, and have actually been saying so and that they should focus more on efficiency for the past few years. I'd rather get a few extra hours out of my phone than have it open apps 0.1 second faster. And, to be fair, the 888 has had repeated reports of running hot and throttling. So I don't think they're in the wrong in why they did what they did, but how they did it was definitely not good.
As @sn3p said, you buy a package, which includes a certain CPU, but it's the OEM that ultimately decides how to make the hardware run. Anybody who spends a decent amount of time on this site knows that's exactly how things work with laptops, with the same chip doing much better in one than another, because they are designed with different TDPs. Why should phones be any different. That's what reviews and benchmarks are for, to see how a phone as a package performs. Nobody with any sense and technical knowledge is going to choose one phone over another simply based on the chip. And as was mentioned by @bruh, if you can't even tell in day-to-day usage, then there's no harm. The only problem is if it only throttles for end users but not in benchmarks, in which case that's a serious problem and a clear case of dishonesty.