You're right that Python 3.15 is not fully released yet, but it's in prerelease which is defined as:

""" After the first beta, no new features can go in, but feature fixes (including significant changes to new features), bug fixes, and security fixes are accepted for the upcoming feature release. """

So it's a fairly well known target. Bruce Eckel published the first edition of this book a quarter of a century ago - he has a pretty good handle on the progression of Python.

Definitions and status for all releases is at https://devguide.python.org/versions/