I'm the guy who started the Cython project and made up the name, so here are some random remarks that don't answer your question. Not only is Cython about 8 years old, it started as a fork of Pyrex (http://www.cosc.canterbury.ac.nz/greg.ewing/python/Pyrex/), which is itself over 11 years old, since I remember using it in 2004 when evaluating languages in which to implement http://sagemath.org. The people who did the most work on Cython on probably Stefan Behnel and Robert Bradshaw (who was my Ph.D. student). I had to sort of twist their arms to get them to be involved in the project, and they both still only do it in their spare time basically for fun. Despite Cython being an incredibly useful and powerful tool for certain specific problem domains (e.g., for SageMath it is critical), and being very heavily used in some scientific python applications, there's no company behind Cython and few people involved in the project are interested in publicity. Cython is an open source labor of love and an extremely useful tool. People that know Cython very well, know how amazingly powerful it is, and have watched many other attempts to solve similar problems come and go over the years.

I definitely agree; as someone who uses Python daily, and previously not knowing what Cython is, my first thought was of a new, very immature project.

The problem is that there is no pattern of version numbers that is meaningfully informative. It's pretty much necessary to figure out how reliable the releases are for each project that you are interested in.

For people that aren't following along, 0.23 might have slightly worse optics than 1.2, but there is plenty of software called 1.2 that is crap.

That's a good point. I'd wonder if it had any production quality seeing that number. Yet, most reviews I read talk like it's beta quality or better for their uses. Hmm.