Does this query always correctly retrieve the types of array parameters
I think you want format_type(typid oid, typmod integer).

Given:

CREATE SCHEMA my_schema;

CREATE FUNCTION my_schema.my_proc_16831(numeric[])
RETURNS numeric[] LANGUAGE sql AS $$
SELECT $1;
$$;


You can get proc details with:

SELECT
  p.parameter_name,
  format_type((p.udt_schema||'.'||p.udt_name)::regtype, -1) 
FROM information_schema.parameters AS p
INNER JOIN information_schema.routines 
  USING (specific_catalog, specific_schema, specific_name) 
WHERE specific_schema = 'my_schema'   
  AND parameter_mode = 'IN'
  AND routine_name = 'my_proc_16831';


... though this removes the portability of using information_schema, since you're relying on Pg specific behaviour. It also assumes the typmod is always -1 because PostgreSQL doesn't keep track of the typmod for functions - e.g. numeric(18,2) becomes just numeric as a function argument.

At least this way you don't need to use your LIKE hack, though, or relying on changeable assumptions about array internal type names.

BTW, you can use pg_proc and pg_type directly instead. This might be worth doing if performance is critical, though otherwise information_schema will be more cross-version portable.

That gives you the equivalent:

SELECT 
  proargnames[argpos] AS arg_name, 
  format_type(proargtypes[argpos],-1) AS arg_type 
FROM pg_proc p 
INNER JOIN pg_namespace n ON (p.pronamespace = n.oid) 
CROSS JOIN LATERAL generate_subscripts(proargtypes, 1) argpos
WHERE n.nspname = 'my_schema'
  AND p.proname = 'my_proc_16831' 
/* AK: suggested additional clause
      AND argpos &gt; 0
*/
    ORDER BY p.proname, argpos;