I get Atmos from every app except Netflix on the Shield 2019.
Open your Netflix account settings on a computer, check the audio playback setting, make sure it is set to high rather than anything lower, then restart the source.
The Prime Video app on the Shield 2019 is not giving me Atmos.
Try turning off Dolby Audio processing on the Shield.
If the problem is not app-specific, the audio capability side is covered in the HDMI audio EDID guide, and the flags themselves are on the MAIN page.
Source: this chapter follows the official DR HDMI 8K user manual (Rev 1.2). Where a setting or value matters exactly, check it against the complete PDF manual rather than an abbreviated web summary.