I don't think people generally write code to use this or another guarantee of the implementation. They write code, they run it, it seems to work (on my machine)

I don't think people generally write code to use this or another guarantee of the implementation. They write code, they run it, it seems to work (on my machine) and that's it. Later, the implementation changes in a subtle way (or someone tries to run the code in an incompatible version) and the code stops working. Nothing to do with "relying on GIL". The same may apply to relying on dict key order, timing of things, or anything else. That's why we have race conditions, Docker images, freezing dependencies, "only works in IE", etc. Writing to spec is very rare in my experience. You usually learn the spec once someone tells you your code doesn't work on XYZ.