tl;dr: Chrome will start blocking 'Set-Cookie' request headers on outbound fetch requests.
The getAll() interface for Headers requires special complex handling of the Set-Cookie header, as it cannot just be combined, since it is semantically a response header. By forbidding 'Set-Cookie' as a request header, we avoid leaking this complexity into requests, as it is not useful for requests anyway. (spec discussion)
There was a UseCounter (results) that was added to verify the usage of 'Set-Cookie' header on outbound fetch requests in Chromium. It was concluded that the % of pages that set a "set-cookie" header on an outbound fetch request hovers around 0.0003%. Further testing indicated that the two most popular domains that set the 'Set-Cookie' header on outbound fetch requests, still continue to work fine when all outbound 'Set-Cookie' headers were removed (i.e. return 200 status codes, identical data, identical response headers). As such, it was concluded that we could safely make this change.
This is technically a web-facing change, though a small change to bring the Chrome implementation in line with the Fetch spec (spec PR). The impact seems small (as indicated by the UseCounter), and only positive by reducing complexity.
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