I think they use something called “flow” last time I’ve checked, it’s like typescript but when I went to visit the website with more information it was so slow it crashed my browser.
You may be thinking about React itself, not the new compiler? I'm sure there must be some flow in there still from back in the day.
I just checked out Flow and woah. First-class syntax sugar for React. Maybe cool. (If it doesn't break catastrophically in a sane build system like Vite)