Mentat

Unfortunately the conditions for automatically updatable views are not met in the discussed case. As documented in https://www.postgresql.org/docs/12/sql-createview.html#SQL-CREATEVIEW-UPDATABLE-VIEWS , such views can only select from a single table (so no JOINs or UNIONs).
The VIEW can be made updatable anyway, using a TRIGGER:

CREATE VIEW events_view AS SELECT ... FROM events INNER JOIN events_json on events.id = events_json.id;

CREATE FUNCTION events_insert_decompose() RETURNS TRIGGER AS $$
    BEGIN
        INSERT INTO events VALUES (NEW.id, ...
        INSERT INTO events_json VALUES (NEW.id, NEW.event);
    END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER events_insert
    INSTEAD OF INSERT ON events_view
    FOR EACH ROW
    EXECUTE FUNCTION events_insert_decompose();

After deep consideration we decided to abandon this approach.