Just one final design philosophy comment from me (I know it isn't needed since Guido already ack'ed the suggestion):

As far as the lessons learned from the historical startswith() case go, avoiding taking up the positional slots with optional flags and configuration parameters is one of the big reasons we added keyword only arguments (with the other being readability at the call site).

I agree we need to be cautious with API complexity, I just think in this case clean handling of empty iterators is a net win (so long as we rule out the conceptually broken case of combining the new parameter with multiple positional arguments).