We can retrieve meta values for custom posts, users, comments, and taxonomies (tags and categories), by querying fields metaValue (for a single value) and metaValues (for an array of values) from the corresponding type:
We can also filter results for custom posts, users, comments, and taxonomies (tags and categories) based on meta, via the input field metaQuery.
For both querying and filtering, meta values may be public or private. For instance, user meta includes entry wp_capabilities, which is private, while description is public. And then there is last_name, which may be public or private depending on the application.
The GraphQL API plugin does not impose which meta keys are public and which are private; this must be explicitly configured. By default, the list of allowed meta keys is empty.