But they do mention that you can disable it at anytime, only that for the telemetry that is sent with the installer you have to set the flag beforehand (obviously):
> The .NET SDK telemetry feature is enabled by default. To opt out of the telemetry feature, set the DOTNET_CLI_TELEMETRY_OPTOUT environment variable to 1 or true.
> A single telemetry entry is also sent by the .NET SDK installer when a successful installation happens. To opt out, set the DOTNET_CLI_TELEMETRY_OPTOUT environment variable before you install the .NET SDK.
> The .NET SDK telemetry feature is enabled by default. To opt out of the telemetry feature, set the DOTNET_CLI_TELEMETRY_OPTOUT environment variable to 1 or true.
> A single telemetry entry is also sent by the .NET SDK installer when a successful installation happens. To opt out, set the DOTNET_CLI_TELEMETRY_OPTOUT environment variable before you install the .NET SDK.