Apple has some very strict rules about who can play in its Apple CarPlay sandbox. That high bar means it took until now for a third-party messaging app to make it through.
WhatsApp has become the first non-Apple messaging app to enter the CarPlay universe. You could use WhatsApp in the car in the past by being a jerk and using your phone while driving, but now it's available as an icon right there on CarPlay's home screen.
Using WhatsApp in CarPlay is just like using the standard Messages app -- you click the icon, and it reads you any unread messages, followed by a prompt that allows you to respond via dictation. New messages will throw up a push notification, too.
While it's pretty well fleshed out, there's one big thing you can't do with WhatsApp in CarPlay. Sadly, you won't be able to scroll through a list of message threads to find a specific one, but that seems to be the biggest drawback.
Nevertheless, it's great to see another messaging app get approval from Apple for use in CarPlay. Does that mean more are on the way? If they're willing to play by Apple's rules, the answer is, "Probably."