To my knowledge, most of those articles were written by face-to-face and old postal hobby players who have probably only ever played the classic variant (not technically a variant, but the original version of the game) and consider variants to be impure forms of the game. However, it is true that only when diplomacy went online has there even been the option to really play-test and develop a variant well, so the "it's just too new" theory could hold some weight.

However, many articles, such as those about negotiation, are applicable to any variant. Likewise, the flow of the classic map can be applied to many other variants because they are often based on it (why do you think there are so many seven player variants?). So maybe the existing articles are still helpful?

But none of this should stop there from being literature on other variants. Perhaps theoretical guides on large variants? Small variants? More specific pieces on the most popular variants? 1v1 tactics? There are countless subjects that are as of yet untapped (to my knowledge at least). And maybe vDip could be the place for that?

I know Enriador has written strategy about Canton in Diplomacy World. I've also read stuff on 1900 and Ancient Mediterranean. The real issue is that there isn't a place on the web that is a repository for all of the information that is produced. A library as such. Everything is just scattered.

i know of a blog who has some articles on 1v1 variants of France vs Austria and Germany vs Italy: https://captainmeme.wordpress.com/

as someone who has played these variants a fair amount, i think the articles don't go into enough details

You'll find articles on Diplomacy (valid for any variant) on the blog brotherbored.com

Check the series "why did I get backstabbed?" (five articles) and "diplomacy is like onions" (three layers)

Thanks CCR. I'll have a look.