We've speculated that Google may be shaking up navigation gestures in Android Q to make up for the bland and boring system it revealed with Android Pie. Companies like OnePlus and Xiaomi have used their own gestures for awhile, and their mature offerings made Google's attempt look a little weak. This hidden gesture discovered by XDA member paphonb seems to indicate that Google is experimenting with a new side-scrolling feature to move forward and back. You'll see what I mean if you watch this short video made using the Android Q emulator.
If you thought about how this would potentially break the existing menu fly-out gesture that apps using Google's best practices for Material Design implement, you're not alone. This new feature would need to be implemented in a clever way to allow apps with the "Hamburger menu" to function as designed.
There's a reason these are hidden in the Q beta builds, but I don't think they are there to stay as-is. Breaking the functionality of apps from the Play Store is something Google hates to do, especially when those apps have implemented what Google called best practices just a few months ago. I do think the developers have a grander idea in mind here and this is just a preliminary test to see what gets broken, how bad it breaks, and what needs to be done to fix it; otherwise these gestures wouldn't be hidden behind adb commands.
Something needs to be done to "fix" Android's gestures, and this is where it starts.