The suit exists because they engaged in a few deceptive practices which people didn't understand. The initial story, which isn't the whole truth but did happen, is that Apple slowed down their phones (to delay the boot problems) but didn't tell anyone. When people complained about their phones being slow, Apple told them to buy a new one rather than admitting what had been done. By the way, we aren't talking about devices that were six years old when this started happening. The batteries would reach the level of degradation where the throttling was enabled in about a year. This leads us to the next deceptive practice.
The batteries they put into affected devices were not good enough. I don't think this was deliberate, although some have alleged that it was, but I assume Apple specified batteries that couldn't provide enough additional voltage to run their boards. Had phones started failing as quickly as they did, a lot of people would have been covered by the warranty, including everyone who had the extended Apple Care. By slowing down the CPU without telling people, they were able to delay the problem until that coverage expired. I owned one of these devices and never disabled the CPU throttling. It still eventually got to the point of repeatedly crashing, rebooting, going from 60% battery to 4% battery in five minutes, etc. It just took a bit longer to reach that point. Does this happen to any old device you have? I've used laptops to the point where their batteries last very little time, but they still don't spontaneously fail like that. People have suggested that this was Apple's attempt to avoid warranty claims for another hardware failure, something that Apple has done repeatedly.
I don't think they were malicious about this or knew from the start that they were going to limit their phones in that way. However, as Louis Rossman has said, when Apple repeatedly has hardware flaws and avoids warranty requirements on them, and each of them just happens to earn them money, then it becomes reasonable to ask whether it was deliberate or if they have failed in their duty to their customers. There is consumer protection law to handle this in a lot of countries.