My solution last weekend when I saw this post was to downgrade both my sheilds to 8.2.3, I never liked the adds on the sheilds main homepage tbh but have gotten used to the as time passed.

In this process though once I downgraded I noticed the adds displayed once again on both my sheilds. This was because the android tv launcher app downloads a update. To get around this I disabled auto upgrade of apps on my sheilds and denyed the upgrade of the Google tv launcher and removed the update. No more adds on the launcher.

I have a feeling this is how google will push full screen adds through their launcher app in the play store.

Here is a well written guide how to downgrade to 8.2.3 that I followed.

https://florisse.nl/shield-downgrade/

Took about 30min on my second sheilds once I figured out the first sheild after 2 hours.