From what I can tell, the ApiResponse objects can never have an undefined config.

Config is defined as optional here:
https://github.com/infinitered/apisauce/blob/master/apisauce.d.ts#L44
But is required here:
https://github.com/infinitered/apisauce/blob/master/lib/apisauce.ts#L200

What problem does this feature solve?

In the past, Brotli was not supported on nodejs servers.

If I understand correctly, this is now supported since a few months : nodejs/node#20458

This PR dropped support for Brotli : nuxt-community/axios-module#180

Maybe it's time to revert this BR because Axios is supporting it too : https://github.com/ax