https://www.mediawiki.org/wiki/Parsing/Notes/Wikitext_2.0/Typed_Templates is relevant in this context (not necessarily the proposal since that is half-thought out as a strawdog, but the discussion at least).

https://en.wikipedia.org/wiki/User:Cscott/Opportunities_for_Content_Transform_Team mentions both Subbu's Typed Templates proposal as well as a number of other possible initial clients/use cases.

T156876: Structured data side channel for wikitext is related. My proposal is to encode the structured data via TemplateData annotations, as described at T156876#11033397:

My inclination at the moment is to say that TemplateData is a good mechanism. Editors already know how to insert templates, and templates form the "microdata" mechanism of wikitext already. If you use {{author-birthdate|1976-09-27}} you're both allowing/enabling alternate styling of that information (including removing it from display entirely) as well as linking the TemplateData of author-birthdate to the parameters you've provided. If author-birthdate says (in its TemplateData) that argument 1 encodes https://www.wikidata.org/wiki/Property:P569 of its subject (the wikidata item associated with {{PAGETITLE}} by default, but presumably could be overridden by an optional second argument), we're most of the way there w/o using anything that editors aren't already familiar with.