In addition to what the sibling comments said, note that "Python GIL release" is a very high level concept and how the GIL is implemented can change over time.

In addition to what the sibling comments said, note that "Python GIL release" is a very high level concept and how the GIL is implemented can change over time. It interacts with how the OS implements locking. FJCVTZS on the other hand is a (mostly) pure function with well defined inputs and outputs, and will likely be useful for Javascript implementations for a long time. Will JS be around in 10 years? Very likely. So ARM adds an instruction for it. The people at ARM have likely put a lot of thought behind this, trying to find the places where they, as instruction set vendor, can help making JS workflows easier. And btw, I'm pretty sure that if python has some equally low hanging fruit to make execution faster, and python enjoys equally large use, ARM will likely add an instruction for it as well.