When I wrote this article first time, only Azure Function V2 was available (legacy now), then after sometime V3 was released. My recommendation was from the options available for V2 only at that time.

Today, V4 is also available which supports .NET 6.0/.NET 5.0 and its configuration options, and you can use IConfiguration (same as .NET 6.0 web/api app).

Now Azure Functions also supports built-in dependency injection same way .NET 6.0 web/api app supports which was not supported in legacy versions.

https://docs.microsoft.com/en-us/azure/azure-functions/functions-versions

Read this: https://docs.microsoft.com/en-us/azure/azure-functions/functions-dotnet-class-library?tabs=v2%2Ccmd#environment-variables