In any case, if one of those packages is at fault, what's the fix?

This is because the iisnode module itself is using the Buffer() method, you may need to update the Interceptor.js file. Simply refer to this thread:https://github.com/Azure/iisnode/issues/60

If you simply don't want to see these warning messages, you can disable them via configuration. Something like:

<iisnode nodeProcessCommandLine="your/path/to/node.exe --no-deprecation --no-warnings"/>