I see what you mean.
I refactored the code in v5 attached and it should now be strict and use double quotes for those scenarios. Additionally, it takes care of the -1 OID issue.
The output of your examples using double quotes:
postgres=# SELECT pg_get_trigger_ddl('"S"."T"','trg');
pg_get_trigger_ddl
---------------------------------------------------------------------------------------------
CREATE TRIGGER trg BEFORE INSERT ON "S"."T" FOR EACH STATEMENT EXECUTE FUNCTION "S".trgf();
(1 row)
postgres=# CREATE TRIGGER "TRG2" BEFORE INSERT ON "S"."T" FOR EACH STATEMENT EXECUTE PROCEDURE "S".trgf();
CREATE TRIGGER
postgres=# SELECT pg_get_trigger_ddl('"S"."T"','TRG2');
2025-10-16 14:03:38.910 CDT [81664] ERROR: trigger "trg2" for table "T" does not exist
2025-10-16 14:03:38.910 CDT [81664] STATEMENT: SELECT pg_get_trigger_ddl('"S"."T"','TRG2');
ERROR: trigger "trg2" for table "T" does not exist
postgres=# SELECT pg_get_trigger_ddl('"S"."T"','"TRG2"');
pg_get_trigger_ddl
------------------------------------------------------------------------------------------------
CREATE TRIGGER "TRG2" BEFORE INSERT ON "S"."T" FOR EACH STATEMENT EXECUTE FUNCTION "S".trgf();
(1 row)
and for -1
postgres=# SELECT pg_get_trigger_ddl(-1,'trg');
pg_get_trigger_ddl
--------------------
(1 row)
