This feature is actually part of Android 8.0 Oreo which also offers up PIP for other apps, although it was largely up to developers to implement it, which Google has done for Google Maps. It was added to the app a few days ago, but it seems that it might have been a soft launch as users were reporting issues, but a recent post on Google’s Product Forums (via Android Police) has suggested that a full rollout could be on its way.
The post by Pixel Community Manager Orrin reads, “A couple of you have mentioned that you are having issues accessing PiP while using Google Maps. Maps PiP functionality is rolling out over the next couple of weeks, and everyone should be able to access it soon by hitting the home button during navigation.”
Obviously this will require users own an Android 8.0 Oreo handset, which means that for now only the Pixel handsets will be able to take advantage of this feature, with other handsets like the Nexus 6P set to receive their update the following week.