APIs.json is a spec for indexing APIs, like how sitemaps work for websites.
I love the idea, but it seems like the project has slowed down, partly due to the interaction of APIs and AI? API Evangelist wrote an interesting article; Twitter and Reddit APIs Are the Canaries in the Public API Coal Mine.
I agree, those were two of the most important APIs and what made those companies valuable, both to profit wise and culturally. It will be interesting so see how that plays out.
Regardless, I think there is still value in a format for documenting and finding APIs, that's primarily hosted on the API owners site.