The point of the "Changed in version ..." or "New in version ..." bits in the documentation is to alert readers who maintain software which needs to remain backward compatible with older versions of Python. If you maintain a package which you support for Python 3.4, 3.5, 3.6, and 3.7, you'll probably shy away from bits which weren't around for 3.4, and be careful about APIs which have changed since 3.4.
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