The use of HTTP 400 dates back to T35646, which was to address
caching proxies and mobile browsers incorrectly caching bad titles
as valid content. However, this also means that caches in front of
MediaWiki, like Varnish, don't cache it either. Since we know that
these titles will always have no content, having these get cached
in Varnish is totally fine. Presumably the 404 will be enough to
tell other crawlers or scrapers that there's still no content on
these pages.
There's some room for debate on whether a HTTP 400 or 404 is more
technically correct here, but emitting a 404 seems like the more
pragmatic option.
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