Perhaps it has to do with supply chain problems during the pandemic?
Doubtful, because they're still offering the Phase IIIs on other models, and they probably source the Klusons from the same manufacturer (most likely Gotoh).
During the early years of the McCarty, Paul Smith stated that he believed that the thinner headstock and Kluson-style tuners sounded better on that model. He's said that he thought the Kluson style tuners sounded better on the 594. They also used them on the earlier Singlecuts. I had several examples of each at the time, and loved the way they sounded.
Not that the more recent ones sound any worse, mind you!
They've gone back to the Kluson type on the entire McCarty line, including the 594, the Joe Walsh model, and I think the hollow 594s as well.
Personally, I have no opinion on whether they sound better or not. I trust Paul's ears, he's been better at designing guitars than just about anyone else. My McCarty Wood Library Trem came with lockers and an "unobtainium" not, instead of a bone nut and the Kluson type tuners, because it's a trem.
In a way, I was relieved, because I hate changing strings on any guitar, and the locking tuners are easier to dick around with. If it had come with the Kluson style tuners, however, I wouldn't have minded. I'm easy.