Just had to add a workaround to #Fedify for http://joinmastodon.org/ns, a JSON-LD context URL that has never actually served a JSON-LD document. Mastodon has always inlined the term definitions, but some implementations put it as a bare URL in their @context, so Fedify's JSON-LD processor tries to fetch it and gets a 404 Not Found. Now Fedify ships a bundled copy of a context that never existed in the first place.
If you have a fediverse account, you can quote this note from your own instance. Search https://hollo.social/@hongminhee/019cff88-7329-7bae-af27-93c248a6c0c6 on your instance and quote it. (Note that quoting is not supported in Mastodon.)