In practice, the choice between using JSON-LD or Microdata might depend on the way websites/emails are built (i.e., one way or the other might be more convenient or not easily possible).
So choices are maybe:
A1) Developers SHOULD use JSON-LD
A2) Developers MUST use JSON-LD
More simple (only one serialization to support
B) Developer MUST use JSON-LD or Microdata
Leaves Microdata as an option where JSON-LD is not feasible
The common knowledge representation language for adding structured data to websites or emails is RDF (https://www.w3.org/TR/rdf11-concepts/).
RDF can have various forms of serialization, such as:
Structured data to websites or emails currently allows for JSON-LD and Microdata (Websites also for RDFa). For websites, Google is recommending JSON-LD (https://developers.google.com/search/docs/appearance/structured-data/intro-structured-data#supported-formats).
In practice, the choice between using JSON-LD or Microdata might depend on the way websites/emails are built (i.e., one way or the other might be more convenient or not easily possible).
So choices are maybe:
A1) Developers SHOULD use JSON-LD A2) Developers MUST use JSON-LD
B) Developer MUST use JSON-LD or Microdata