Potentially undocumented behaviour change in Postgres 11 concerning OLD record in an after insert trigger

Hi,

I've noticed a change in the behaviour in triggers / hstores in Postgres
11.1 when compared to Postgres 10.5.
The following won't work on Postgres 10.5 but in Postgres 11.1 it works
just fine:

CREATE EXTENSION hstore;

CREATE TABLE _tmp_test1 (id serial PRIMARY KEY, val INTEGER);
CREATE TABLE _tmp_test1_changes (id INTEGER, changes HSTORE);

CREATE FUNCTION test1_trigger ()
RETURNS TRIGGER
LANGUAGE plpgsql
AS
$BODY$
BEGIN
INSERT INTO _tmp_test1_changes (id, changes) VALUES (NEW.id, hstore(OLD) -
hstore(NEW));
RETURN NEW;
END
$BODY$;

CREATE TRIGGER table_update AFTER INSERT OR UPDATE ON _tmp_test1
FOR EACH ROW EXECUTE PROCEDURE test1_trigger();

INSERT INTO _tmp_test1 (val) VALUES (5);
ERROR: record "old" is not assigned yet
DETAIL: The tuple structure of a not-yet-assigned record is indeterminate.
CONTEXT: SQL statement "INSERT INTO _tmp_test1_changes (id, changes)
VALUES (NEW.id, hstore(OLD) - hstore(NEW))"
PL/pgSQL function test1_trigger() line 3 at SQL statement

I couldn't find anything about this in the release notes (
https://www.postgresql.org/docs/11/release-11.html), but maybe I just
didn't know what to look for.