Since Google won't be selling them directly through the wireless service, they might not support things like fast network switching between T-Mobile, Sprint, and Wi-Fi, or even enhanced network features.
Google Fi ditches the old yellow and green color scheme in favor of a combo of Google's iconic green, blue, red, and yellow hues and now has plenty of the fancy Google Sans font everywhere you look.
Google's Project Fi wireless service first launched back in 2015, offering people in certain regions of the United States a new and unconventional alternative to traditional cellular service. Technically, Fi always worked on phones that weren't designed for it - you'd just miss out on being able to use multiple networks if you put a Fi SIM card in unsupported hardware. And the new service now supports iPhone and most Android phones, addressing Fi's only major drawback. Google spoiled this a bit by sending out "Google Fi" flyers (complete with the new logo) with some Pixel 3 purchases earlier this month, but now it's official.
Update: Google's blog post is live now too.
Keep in mind that just because a phone is compatible with Google Fi means that it gets every Google Fi feature. Phones that haven't been optimized for Fi won't auto-switch to Google's Wi-Fi, the same way they won't auto-switch through the three carriers. I'm not sure why you would sign-up for Fi if you can't get those features, but OK.
With today's announcement, however, none of the aforementioned issues should crop up anymore. But the Google Fi website has gone live, and it explains what's changed. Google's expanded device support will instead allow any normal phone to work on Google Fi as a purely T-Mobile device.
Speaking of which, Google is ringing in the Fi relaunch with two travel-themed promotions, both of which run for today only. As of this moment, you can visit the Google Fi site and sign up for service using most Android phones; conversely, iPhone owners are directed to get started with the Google Fi iOS App - though you should note that Fi for iPhone is launching as a beta. If you bring your own device to Fi, you'll get a $200 service credit.