Android 9 Pie update reportedly coming to the 2017 NVIDIA Shield TV0
However, it looks like there's a high chance that at least one of the NVIDIA Shield TV iterations will be updated to Android 9 Pie. Spotted by @AndroidTV_Rumor on Twitter (via 9to5google), the listing of the NVIDIA Shield TV on the Google Play Developer Console mentions that the Shield TV is compatible with Android Pie.
Unfortunately, the 2017 NVIDIA Shield TV is the only one mentioned in the Google Play Developer Console, so it's unclear whether or not Android Pie will come to the older models as well. Although the 2015 NVIDIA Shield TV has almost the same hardware as the newer version, there's no guarantee that it will get the update.
If you don't own an NVIDIA Shield TV yet, you might want to stay your purchase, as the company is expected to launch a revised version with Android Pie and a slightly better processor inside.