Embedded "microformats" like JSON-LD (or, say, Dublin Core RDF metadata) are somewhat problematic, in that they're not covered by the HTML standards, and not documented on any of the "HTML5 tutorial" sites—since they're something "above" HTML. (It'd be a bit like expecting such sites to document how to write inline SVG in your HTML5 document.)
So there's no "effortless" way for knowledge of HTML-embedded microformats to filter down to most HTML document authors or web-app theme creators. They have to actually go and seek such knowledge out. And often they don't know what they don't know†, so they have no reason to.
Essentially anything on the web that requires or relies on an HTML-embedded microformat to work, is instantly relegated to being an "SEO black art" that just gets passed around in blog posts read by bigcorp web-app devs and no one else.
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† This is one big part of what the people who dislike minification, WASM, etc. are complaining about re: the "readable" web. It used to be that you'd discover techniques like this by just looking at a bigcorp's production web pages and seeing how they work. That's far harder today than it used to be, and it's only going to get even harder in the future.
RE: the last point, that doesn't seem so. Desktop apps have always been opaque, same as server backends. If anything, the rise of SPAs, WebAssembly, Service Workers and JS in general means you can see a lot more of the action in your browser rather than it being hidden away somewhere.
WebAssembly expressly has a textual representation so that it can be inspected, it's part of the spec. And for everything else that's compiled, there are always decompilers to give you the source, often times at great quality.
HTTP/2 is binary (and by default encrypted) but nobody notices because browsers and tools transparently convert it to text for you, even though there were plenty of complaints that it would be hard to inspect.
So there's no "effortless" way for knowledge of HTML-embedded microformats to filter down to most HTML document authors or web-app theme creators. They have to actually go and seek such knowledge out. And often they don't know what they don't know†, so they have no reason to.
Essentially anything on the web that requires or relies on an HTML-embedded microformat to work, is instantly relegated to being an "SEO black art" that just gets passed around in blog posts read by bigcorp web-app devs and no one else.
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† This is one big part of what the people who dislike minification, WASM, etc. are complaining about re: the "readable" web. It used to be that you'd discover techniques like this by just looking at a bigcorp's production web pages and seeing how they work. That's far harder today than it used to be, and it's only going to get even harder in the future.