It's true that Toyota does employ NiMH batterys often, but they still offer models with Li-ion (Prius Prime, FWD Prius). The NiMH batteries tend to be found in the cheaper models (Prius C/Aqua) or those intended for colder climates (Prius AWD).
Edit: This article said that the new nickel is 1000Wh/L meanwhile Li-ion on cheaper model is 740Wh/L. It seems that now nickel overcomes Li-ion for Wh per volume. (other Li-ions also seems to <= 800Wh/L)
https://car.watch.impress.co.jp/docs/news/1338990.html
https://www.eenewspower.com/news/nanograf-hits-800whl-milest...
Both types of batteries have pros and cons - this YT channel of a Toyota mechanic provides pretty clear explanation of the differences https://www.youtube.com/watch?v=AS3Lg3qJcnc
Pretty much one, and only remarkable trait is that they work well at < 0C°