Iris 2.4 is a final extra release of Iris 2, which back-ports specific desired features from Iris 3 (not yet released).
The purpose of this is both to support early adoption of certain newer features, and to provide a final release for Python 2.
The next release of Iris will be version 3.0 : a major-version release which introduces breaking API and behavioural changes, and only supports Python 3.
For more details see the What's New page: https://scitools.org.uk/iris/docs/latest/whatsnew/2.4.html