I do not get the point of the information_schema

I try to implement SCD2 on trigger level and try to generated needed
code on the fly. Therefore I need to read data about the objects in the
database. So far so good. I know of the information_schema and the
pg_catalog. The documentation for the information_schema states that it
'is defined in the SQL standard and can therefore be expected to be
portable and remain stable'. I can think of a sensible meaning of
portable. One cannot port it to MariaDB, can one? Maybe different
PostreSQL version but then a one fits all implementation would mean only
parts of the catalogue that never ever change can be exposed by the
information_schema. Coming from Oracle I consider the information_schema
the analogy to Oracles data dictionary views giving a stable interface
on the database metadata hiding catalogue structure changes. But I
dearly miss some information therein. I created following query to get
the index columns of an index. I fear breakage when not run on the
specific version I developed it against. Is there a more elegant way by
the information_schema?

with INDEX_COLUMN_VECTOR as(
select
i.indkey
from
pg_catalog.pg_index i
inner join pg_catalog.pg_class c on
i.indexrelid = c.oid
where
c.relname = 'idiom_hist'
),
COLUMNS as(
select
a.attname,
a.attnum
from
pg_catalog.pg_attribute a
inner join pg_catalog.pg_class c on
a.attrelid = c.oid
where
c.relname = 'idiom'
) select
c.attname
from
COLUMNS c
inner join INDEX_COLUMN_VECTOR v on
c.attnum = any(v.indkey)
order by
c.attnum asc;

An other simpler case.

select
indexname
from
pg_catalog.pg_indexes
where
schemaname = 'act'
and tablename = i_table_name
and indexname = i_table_name || '_hist';

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