It seems to be nearly impossible to find good resources about the subject as they have been crowded out by AI-related SEO-sites. But have you started with W3C’s specs on the subject? Ontologies, linked data, RDF, etc. In Norway we have a government agency publishing local adaption and guidance for these concepts and standards, so that is at least two sources of information on the general concepts and syntax at least. What are you looking for?
I found the same - it's definitely studied in academia, some people do seem to practice it, and there do seem to be some standards or frameworks in some places.
Besides AI/LLM the linked data ecosystem struggles with a deep adversion to the technology by the general developer population, dating back from the time when the Semantic Web hype cycle fizzled out. You can see that for instance very clearly on the Fediverse, where the ActivityPub [0] protocol is a W3C linked data based open standard. But ActivityPub can also be plain JSON, and most Fediverse software does not follow real JSON-LD approach. There's a lot of resistance to going the linked data route, being perceived as overly complex. Which is probably a fair assessment too, as the whole set of standards around linked data are large and intricate. As a result of Semantic Web and Linked Data failing to find good adoption, there's a chicken/egg that there's no good tooling to work with the technology and little incentive to create it.
[0] https://www.w3.org/TR/activitypub/