How about Turtle

[VladimirAlexiev]

Subject says it all.
(Is it common to define ontologies in a JSONLD context? I didn't know that)

[sballesteros]

Note that the ontology is not really defined in a JSON-LD context. We just use JSON-LD as one of the possible serialization of RDF. For instance, we also provide an RDFa serialization (view source on http://ns.science.ai). I am not sure how common that is. Out of memory i can think of https://www.hydra-cg.com/spec/latest/core/#the-hydra-core-vocabulary-in-json-ld as another example.
We would be happy to provide additional serialization if helpful!

[VladimirAlexiev]

Yes, publish RDF/XML (mandatory) and Turtle (highly recommended), and serve them with content negotiation.
http://rdf-translator.appspot.com does all sort of conversions, and you can automate easily, eg

http://rdf-translator.appspot.com/convert/rdfa/n3/http://ns.science.ai > scholarly-article.n3

n3 is almost the same as Turtle except = should be owl:sameAs.

Buggies:

• sa:Acknowledgements = <http://purl.org/spar/deo/Acknowledgements> .
  Replace with owl:equivalentClass as = (owl:sameAs) is intended for individuals
• http://purl.org/spar/doco/ etc:
  Define and use prefixes for all referenced ontologies. (eg http://prefix.cc/doco says that's doco:)
• @prefix sa: <http://ns.science.ai#> .
  Since your ontology is served by http://ns.science.ai/ (if you agree to fix conneg doesn't work #24), so the prefix should include slash: http://ns.science.ai/#.
  It's a bit unusual to have both slash and hash, but that's ok; unless you're willing to change to eg http://ns.science.ai/ontology#
• rdfs:isDefinedBy "https://ns.science.ai/"@en-us ;
  Instead of literal, you should use a URL. The best practice is to use precisely the ontology prefix:
  rdfs:isDefinedBy sa: ;
• dc:source <https://github.com/scienceai/scholarly-article/issues/20> ;
  Change to dct:source, which is an object property (dc: is merely rdf:Property)
• include rdfs:label for each term.
  In particular, what's the point of providing altLabel if you don't have label.
  Note: unfortunately both skos:prefLabel and skos:altLabel are subprops of rdfs:label, so you may want to use skos:prefLabel for your main labels
• comments and labels use @en-us. Use simply @en because I don't see any US English specific constructs.