To Wizards of the Coast:

We ask that you please allow for us to release the unpublished manuscript of the World of Mystara AD&D 2E campaign setting by Jeff Grubb, which we have a copy of, and which Jeff has approved of us sharing, pending your approval. All we need is a simple "yes." We suggest either letting us post the document at the Vaults of Pandius (the official Mystara fan-site), or for you (WotC) to post it as an item of "historical interest" in a DRAGON+ magazine article, and/or post it for sale at DMs Guild as a D&D Classic PDF.

Thanks for considering,
—World of Mystara enthusiasts

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Update: April 9, 2021:

I wrote to Wizards Customer Service awhile back, and there was no response whatsoever. I also wrote to the publicly listed email address for one of WotC's IP attorneys, and also no response.

I would suggest that fans write to WotC Customer Service directly, and express your wish to see this, as WotC consumers:

https://support.wizards.com/hc/en-us/requests/new

Update June 10, 2019:

After reviewing our petition, Jeff Grubb sent a note: 

I had only a hard copy, and sent it to [the webmaster of the Vaults of Pandius]. [...] If you succeed, I will be glad to provide an intro with a less-depressing history of the project.
-Jeff

So if our petition succeeds, Jeff has offered to write a brand-new introduction to the 2E Mystara worldbook!

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Here's the background story:

Back in 2017, veteran game designer Jeff Grubb blogged about why he left TSR. He says that the straw that broke the camel's back was how the AD&D 2E worldbook for Mystara was nixed after he'd already written it.

Here's his initial post: "Why I left TSR".

And here's his overview of the manuscript: "Mystara Redux".

In the manuscript, there are 54+ new kits for AD&D 2E! This is a huge expansion of the 2E kit system, and setting-wise, this document would serve as a resource for any eventual 5E presentation of the world. Some of the terminology* is unique to this document, and so synthesizes the D&D world of Mystara in a way that's never been seen before, thus significantly adding to Mystaran lore.

*(Such as the "Undersiders" kit for Rockhome Dwarf rogues.) 
***
Toward this end, we wrote to Jeff Grubb (in September 2018), and he kindly replied:

Thank you for you note. Dead and unfinished projects litter the backstory of any creative, but Mystara's passing made me particularly sad. I'm afraid I already sent out my printout of the design to [the webmaster of the Vaults of Pandius, the official Mystara fansite], who asked about it a year ago. You may be able to get a copy from him. Should I come across anything else in my files, I will send you a note.
Best of luck,
Jeff Grubb

***
Update: The webmaster who holds the manuscript sent us a kind note:

That's a great offer from Jeff.

Apologies that I never got back to you to update you on my attempts to get back in touch with Wizards - however the main reason for that is because it went frustratingly nowhere.  There had been no response, not even an acknowledgment of receipt of email.  I was frustrated but I still didn't see it as a reason to "just release it".

I appreciate that your petition is towards Wizards to release the manuscript and I look forward to a positive response from them in that regard.

Yes, the rather large manuscript, is only in hard copy - but I don't see that as a hindrance to moving forward in the case of a positive response from Wizards.

—Shawn Stanley (webmaster with the Vaults of Pandius - The Official Mystara Fan-Site)

***

So that's where we stand.

Whether you're a BECMI Known World grognard, or from a newer generation who's intrigued about the mentions of Mystara in the 5E books, would you consider signing and sharing/tweeting this petition?

Update: Given WotC's non-response, I'd suggest also writing to WotC directly, and making your opinion known: https://support.wizards.com/hc/en-us/requests/new