You could, of course, create a statement like "const len" to flag that len will NOT be changed, thus creating true backwards compatibility,
Somehow this idea is getting tangled in my mind with the distinction between mutable and immutable objects. When you use an object as a dict key, it must not change, in order to allow the optimization that keys can be sought through hashing rather than by sequential search. Similarly, making a name such as True immutable allows the optimization that "while True:" can be determined during compilation to be unconditional.
I understand that there is a difference between the kinds of immutability, but still there seems to be a strong connection here.