Drupal core can now generate JSON Schemas for content entity types.
The typed data, serialization and field APIs have been enhanced to allow field-level schemas to be generated based on their storage configuration.
All field types shipped by core now provide JSON Schemas out of the box through their default normalizers. In addition, all the core typed data plugins provide JSON Schemas as well. This means that all core fields can generate JSON Schemas for their properties out of the box. Additionally, most field types provided by contrib or custom modules will generate JSON Schemas automatically so long as they do not provide a custom normalizer or depend on non-core typed data plugins.
While Drupal Core now contains an API for generating these schemas for content entity types, site-wide schema generation in an end-user format (e.g., OpenAPI) is left to contrib and custom code. The openapi_jsonapi module uses this new functionality to provide an OpenAPI schema document for sites using JSON:API.
Expert mode
In a case where custom or contrib code does implement a custom normalizer or typed data plugins (this is rare), they can be updated to provide JSON Schemas. If a higher-level core normalizer attempts to ascertain a JSON Schema from a class which does not support it, a no-op schema containing only a comment will be generated.
Custom typed data plugins which provide their value through the TypedDataInterface::getValue() method may simply add the new JsonSchema attribute to that method.
If a different method is used (see PrimitiveDataNormalizer for example, which calls PrimitiveInterface::getCastedValue()) see PrimitiveDataNormalizer::getNormalizationSchema() for an example of a more complex pattern.
In only very rare cases will non-core code need to configure a normalizer explicitly for JSON Schema support; see FieldNormalizer and FieldItemNormalizer if you veer into this expert territory.
