Since you are asking various "meta" questions, let me explain the wider context first.

This bug report is coming from SageMath. Currently, SageMath only supports Python 2.7 but we are slowly and steadily porting it to Python 3. This bug is one of the many issues that we found while working on this port.

It could very well be that SageMath is the only Cython project which *installs* the source code (although it would make sense to do that for all Cython projects once this bug is fixed). So the fact that nobody has complained before is simply because the only project that could hit this bug is running on Python 2 only.

Also, people may be so used to not seeing Cython code in tracebacks that they don't question it. They may think that it's just a fact of life that it doesn't work.

Concerning backporting to 3.y.z, that is not my decision to make. Of course, I would like to see this backported (it would be trivial to do that), but that should be decided by whoever accepts the pull request. In any case, I don't see why that should influence whether the patch should be accepted for 3.8. That seems to be putting things in the wrong order: first, the patch is applied to 3.8 and then we decide whether to backport it.