I'm guessing the Android Launcher is next to have banner ads.
Also, the default launcher happens to be something heavily customized by each OEM (likely extending the AOSP launcher which will never have ads or more typically extending a shared base launcher[0]), and I think routing ads through OEMs would be both a technical and logistical impossibility.
The phone space is such a competitive market, and the cost of switching (launchers/to a different Android phone/to iOS) is so low, and the risk to future sales so high, I really doubt you'll see ads on the launcher.
[0] https://cs.android.com/android/platform/superproject/+/maste...
Pretty simple to do on the Shield too.