Could it be an idea to also add
OUT can_be_zero boolean
to pg_get_catalog_foreign_keys()'s out parameters?
This information is useful to know if one should be doing an INNER JOIN or a LEFT JOIN on the foreign keys.
The information is mostly available in the documentation already,
but not quite accurate, which I've proposed a patch [1] to fix.
[1] https://www.postgresql.org/message-id/4ed9a372-7bf9-479a-926c-ae8e774717a8@www.fastmail.com