The metAnvil (meta-Anvil) is envisioned as a set of tools, tips, articles, thawts, and tests that use World Anvil as a platform for worldbuilding and creating stories, RPG adventures, maps, and artistic works.
This "world" will take a systemic approach to the "meta" behind making our worlds on World Anvil. We'll look at the tools of the website itself [the Toolbox], the art and science of worldbuilding [the Recipe Box], the things we can do to make the presentation of our world pretty [the Paint Palette], and other meta-type stuff [the Fourth Anvil].
There are a number of articles, websites, books, podcasts, etc. that cover various aspects of building worlds. There is even a "world" or more on World Anvil that tries to break the coding in order to help find and fix issues. The metAnvil attempts to put it all in one place as a reference and to explore how these different areas may interconnect.
At the moment, metAnvil is new and is just a single theorist/practitioner. It's not official. There are other informational "worlds" on World Anvil that are for that (but we have links to those! ). This is more like a collaborative laboratory than a canonical encyclopedia.