Can generally confirm. Not from a "tracking" perspective, but from a "General usability in the modern world" perspective. I spent a year with a KaiOS Flip IV (about a $70 AT&T phone), and finally gave up and went back to a smartphone when the keyboard got so bad I couldn't dial a phone number without massive corrections, much less text.
It felt, first and foremost, like a device built by someone who had been told stories about the phones of the first 5-8 years of the 2000s, but had never seen one, never used one, and wasn't quite sure how it was all supposed to really work. Lots of really weird pain points like predictive text not capitalizing the lone "i" no matter what mode I was in. I started trying to write texts without "I" in them to avoid having to perform the 72 keystroke sequence to capitalize it - I don't like being lazy in texts about things like that.
The performance wasn't bad initially, but it felt like there were some seriously non-optimal data structures involved in everything. A text thread with more than 50 messages or so started being slow to interact with, God help you if it's got more than a few pieces of media in it. And if the phone had too many text messages in general, the whole texting app would lag badly (seconds to open, seconds to see a list of conversations, etc). I had to very aggressively trim old threads, and sometimes even current ones, to try and keep the texting app responsive.
It also got oddly prone to "Oh, yeah! There was a thing I was supposed to notify you about! You're looking at me now, I remember, you got a text! chime" sorts of behaviors I could never run down the cause of. I'd be texting someone, waiting for a reply with the phone closed, and... nothing. Magically, as soon as I opened it 10 minutes later, I'd get the notification that they'd sent a text. I hate to say things like "You have one job," but "Notifying me that there's a phone call or text message" is sort of the core function of a modern phone-type device.
The browser was a joke. I could browse Hacker News and that was about it. Anything more complex would fail to render, or just crash the browser. A modern web capable device it was not.
I think I gave it a fair shake for a year. But I was still doing things like "Carrying an iPhone, powered down, for when I needed something I couldn't do on KaiOS." It was mostly either drone ops or a particular building access app that I needed often enough, so I don't really fault it, but it was annoying not having them on my carry device. Same goes for "literally anything but SMS." Attempts to get a Matrix client on the device didn't go very far (there is one, but it doesn't work very well), Signal was a no-go, and nothing else was useful either.
It did have Bluetooth, though. And I did like being able to cart around music on SD card. Don't carry too much, though, the music app couldn't handle a ton.
I moved to it when Apple decided to push on-device CSAM scanning, and... a year and another OS revision later, they've seemingly not done that. And then iOS 16 added Lockdown, which I decided to play with, determined did everything I wanted in such a feature, and I'm back to an iOS device with very few apps installed. I never had warm fuzzies about the security posture of the KaiOS ecosystem either - Apple, I do generally trust to try to secure their devices. KaiOS, nothing about it really felt robust against a remote attacker, though I've no evidence that it's actually full of holes. It just never gave any real confidence that it was more than a best effort, YOLO'd sort of thing tossed over the fence.
There were some fun little games, but they were all very peppered with advertising. At least you could sideload stuff.
Anyway... it's a thing, I tried it, and despite my exceedingly high tolerance for tech pain, I couldn't make it past a year. Failures on my part, I suppose. I could have probably dismantled it and repaired the keyboard...