> Maybe it would be easier just to remove "A foreign key must reference columns that either are a primary key or form a unique constraint" restriction, stating that any unique index is enough.

The problem is that only btree indexes support uniqueness atm. That's the relevant unsupported features, not the ability to have constraints (which essentially just requires uniqueness support of the underlying index):

    postgres[27716][1]# SELECT amname, pg_indexam_has_property(oid, 'can_unique') FROM pg_am;
    ┌────────┬─────────────────────────┐
    │ amname │ pg_indexam_has_property │
    ├────────┼─────────────────────────┤
    │ btree  │ t                       │
    │ hash   │ f                       │
    │ gist   │ f                       │
    │ gin    │ f                       │
    │ spgist │ f                       │
    │ brin   │ f                       │
    └────────┴─────────────────────────┘
    (6 rows)

Edit: different uses of word constrain (to constrain, and a constraint) seemed too confusing.