For those wondering the new 2020 Google Chromecast with Google TV does support sideloading apps and my Downloader app. There was a small bug in Downloader that partially prevented file access on the Chromecast, due to privacy changes made in Android 10, but I’ve already fixed it and the updated version (v1.4.2) is already live in Google’s appstore.
The only catch with the new Chromecast, which may or may affect other Android TV devices once they get updated to Android 10, is that you must enable developer mode before you’re allowed to enable apps from unknown sources. This isn’t the case for other Android TV devices, like the Nvidia Shield TV, which allows you to toggle on apps from unknown sources immediately.
I’ll have detailed instructions on how to sideload on the 2020 Chromecast up soon, but you essentially have to go to Settings > System > About and scroll to the bottom where you’ll find the device’s build number. Highlight it and keep pressing Select on the remote until it says “You are now a developer!” at the bottom of the screen. Only then can you navigate to Settings > Privacy > Security & Restrictions > Unknown sources to enable Downloader to sideload apps.