Note: Before PostgreSQL 9.6, pg_am contained many additional columns representing properties of index access methods. That data is now only directly visible at the C code level. However, pg_index_column_has_property() and related functions have been added to allow SQL queries to inspect index access method properties; see Table 9-62.
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I'm supposing the structure of pg_catalog objects are deemed implementation details is why they lack of a release note compatibility item - though it does show up in 3.3.12 of the 9.6.0 notes.
