What is needed is a change to the Apache confic for Wikidata. A redirect there is needed from Main Page to Wikidata:Main Page.

the public repo for this is 'operations/apache-config', the ServerName www.wikidata.org can be found in the file main.conf. if a volunteer likes to , have a shot at it and i can test it

You should fixed the interwikis to [[d:]], that will than redirect to the main page in Wikidata namespace automatically. INVALID for me.

@JanZerebecki: What's your take on this?

We should just simply use javascript for this. While it's less reliable than an apache config, it would give the community more control and will be easier when it needs to be modified.
Something like this in common.js would work, I think.

if ( mw.config.get( 'wgPageName' ) === 'Main_Page' )
     {
     window.location.replace("//www.wikidata.org/wiki/Wikidata:Main_Page")
     }

Not in JavaScript please. This needs to work for everyone. And it is not something that needs to change in the future.

Change 181992 had a related patch set uploaded (by Glaisher):
Redirect Main_[Pp]age to Wikidata:Main_Page

https://gerrit.wikimedia.org/r/181992

Patch-For-Review

We just keep the local redlinks red, right?

Yeah I'd say so.
Thanks for making the patch!

In T58258#593894, @duplicatebug wrote:

You should fixed the interwikis to [[d:]], that will than redirect to the main page in Wikidata namespace automatically. INVALID for me.

Agreed. Those titles were never valid. JavaScript redirects may be put in place by the admins, if the community really cares so much, but having redirects from invalid titles is not the ops' job IMHO.

Do we know how many links there are from across the projects to this page? If it's just a few links, should be easy to fix by hand.

I couldn't actually find this URL in use. See https://www.google.ru/#q=%22wikidata.org%2Fwiki%2FMain_page%22+site:wikipedia.org . Could anyone list a few pages already containing it?

In T58258#948897, @JanZerebecki wrote:

I couldn't actually find this URL in use. See https://www.google.ru/#q=%22wikidata.org%2Fwiki%2FMain_page%22+site:wikipedia.org .

I think the search would be https://www.google.it/search?q=link:http://www.wikidata.org/wiki/Main_Page (0 results)
But even https://www.google.it/search?q=link:http://www.wikidata.org/ has only 387 results for me (protocol or subdomain don't matter).

Change 181992 abandoned by Glaisher:
Redirect Main_[Pp]age to Wikidata:Main_Page

Reason:
Per task. It looks like there are no valid links to the page.

https://gerrit.wikimedia.org/r/181992

/me proposes "declined". I can't find any links to this page either (except locally). If anyone sees such links, please fix them.