In this case I'm not sure if we should change the example when I look the examples version in v3.6:

BEGIN
   SELECT DECODE(dummy, a, x 
                      , b, y
                      , c, z
                      , 0)
     INTO l_result
     FROM dual;
END;
/

BEGIN
   l_result := CASE dummy
                  WHEN a THEN x
                  WHEN b THEN y
                  WHEN c THEN z
                  ELSE 0
               END;
END;
/

Maybe we should think about making rule G-1050 more precise and define where literals are acceptable. It's one of those checks that throws tons of violations in any code I analyze. As a result I typically disable the rule completely.

In any case we should try to avoid violating any of our own guidelines in the good example and try to violate only one guideline in the bad example.