There are a couple unique use cases, such as http://tools.wmflabs.org/pageviews/, that want to be able to show redirects just as they are. Supporting this use case in the completion suggester itself might be a pain, but we could offer a way to fallback to the old prefix search.
Do we need it in all API functions, specific API? Because now all APIs go to SearchEngine implementation, so implementations are free to do what they want. We'd have to either force it on API level (calling default one instead of completion one) or somehow control all implementations.
I think for this use case supporting it only in ApiQueryPrefixSearch would be sufficient. I'm not sure the best way of exposing this to api users, most obvious to them would be to have an explicit api flag but i'm not sure yet what it would look like. Needs some consideration.