Following the resolution of T124365, we should clearly document our policy for REST API format versioning. It should be prominently mentioned in https://www.mediawiki.org/wiki/API_versioning, and referenced from Swagger documentations. Ideally, each end point's documentation will encourage users to explicitly provide an Accept header.
The implementation in RESTBase will start with the Parsoid content end points, to support the migration towards separate data-mw metadata (see T78676). Some Varnish work is needed to efficiently vary on Accept headers.
More general validation for Accept headers vs. supported response types might follow at a later point.