Is the POWERSHELL_TELEMETRY_OPTOUT the same Environmental Variable you can define in Group Policy?
Just curious of those of you have tried this. Is the POWERSHELL_TELEMETRY_OPTOUT pulling from Windows Environmental Variables or some other PowerShell specific Environmental Variables?
If this is pulling from Windows Environmental Variables, then you could define this in your Default Domain Group Policy object using Group Policy Preferences (GPP).
See: Computer Configuration\Preferences\Windows Settings\Environment