In meantime this can be hacked in using this in your LocalSettings...

if ( strpos( $_SERVER['HTTP_HOST'], '.m.' ) ) {
$wgMFMobileHeader = "Host";
}
$wgMobileUrlTemplate = '%h0.m.%h1.%h2.%h3.%h4';

Something like this in an apache config should work to set the header:

RewriteEngine on
RewriteCond %{HTTP_HOST} ^[^.]+\.m\.wiki\.wmftest\.net$
RewriteRule ^.*$ - [E=MOBILE]
RequestHeader set X-Subdomain 1 env=MOBILE

This would need template magic to work properly with a non-standard mediawiki::multiwiki::base_domain hiera setting (eg on a Labs host).

I spent the better part of an afternoon working on this. I've come to the conclusion that there is no nice or easy way to implement it in MediaWiki-Vagrant.

In the WMF production environment we do this Host header detection and munging in the Varnish layer. Varnish adds the X-Subdomain header and rewrites the Host header in the request that is eventually forwarded on to the MediaWiki backend host. A pure Apache setup is not capable of munging the Host header for the inflight request in this way.

Why is this still stalled, and normal priority? The blocker was marked as resolved and it has two blocking entries, one of them an unbreak now, the other a high priority task.

This task was made to block T100413, which is UBN!, as it was/is quite difficult for the Reading Web team to verify the behaviour of the proposed changes.

I'd say that this task tracks T100413 rather than blocks it.