Problem/Motivation
I'm sure it's been noticed before, but #2926508: Add DateTimeNormalizer+TimestampNormalizer, deprecate TimestampItemNormalizer: @DataType-level normalizers are reusable by JSON API has shown quite clearly the awkward dance that we do to make "date only" datetime fields. We use datetime fields, but then treat/interpret them as "date only".
This means it depends on @FieldType-level configuration, which implies that core REST/Serialization, and also JSON API, GraphQL, Default Content and so on, need to be explicitly aware of this.
For a crystal clear example, see \Drupal\serialization\Normalizer\DateTimeIso8601Normalizer that is being added in #2926508-81: Add DateTimeNormalizer+TimestampNormalizer, deprecate TimestampItemNormalizer: @DataType-level normalizers are reusable by JSON API.
Proposed resolution
Introduce a new @DataType to represent dates.
Remaining tasks
TBD
User interface changes
None.
API changes
TBD
Data model changes
TBD