> But http.client needs to be able to communicate with any random server
> created since the dawn of time.

Well, that sounds a bit unreasonable...

> Often on 8 bit microcontrollers that haven't been updated since 1994.

Anyone with such needs should write specialized software, shouldn't they?

> How does keeping 0.9 client support in hurt us?

In any such situation, there's typically a long-term cost in additional maintenance when patching and improving the code.
More specifically, this issue came when discussing #6791. Protecting http.client against unbounded reads will be hairy if we have to support HTTP 0.9-style "simple responses".