Google promised an Assistant driving mode for phones would arrive in mid-2019, but that clearly didn’t happen — over a year passed without any sign of it. It appears to be ready, though. XDA-Developers has discovered (via Android Police) that Google Assistant’s driving mode is at least partially enabled for Android users. The interface has changed considerably from the I/O 2019 demo you see above, but the concept remains the same with large buttons and text that let you chat, message and play music while keeping your driving distractions to a minimum.
The rollout appears to be server-side, and might be part of a test. It’s not attached to any particular versions of Google’s Maps or search apps, and also works on a variety of devices. Your access might depend on your account.