missing AZURE_DEFAULTS_SUBSCRIPTION environment var?

I find it strange the default subscription is missing from the defaults section in:
https://docs.microsoft.com/en-us/cli/azure/azure-cli-configuration#cli-configuration-values-and-environment-variables

Was this done on purpose? It makes the `az` cli hard (and a little unsafe) to use in a multiple subscription environment with standard tools such as [direnv](https://github.com/direnv/direnv).