Can you explain what you are trying to do here, isn't it correct to continue to read headers on 101? For example consider this server response (cribbed from wikipedia)

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=
Sec-WebSocket-Protocol: chat

Exactly, the new protocol doesn't actually start until after the set of HTTP/1 headers. Explained in RFC 7230 section 6.7.

But exactly how libcurl should behave in the case of a protocol upgrade to a protocol libcurl doesn't understand has never been defined and documented clearly.

The code that I have changed gets executed after an empty header line (= end of headers) has been read. It's inside the "if" starting at https://github.com/mkauf/curl/blob/c8a077a2ecc12a7b601a00b3531b79c106b4e49e/lib/http.c#L3022

So the k->header flag controls whether curl expects another set of headers.

But exactly how libcurl should behave in the case of a protocol upgrade to a protocol libcurl doesn't understand has never been defined and documented clearly.

That's right... but probably this protocol does not use HTTP headers (e.g. WebSocket)