I seem to remember I traced this back through the pre-github version control, and that is when it was deprecated.
I also seem to have just done it again, and I got this wrong -- I now think it should say **0.6** .  Will fix.
There is a full ongoing list at https://github.com/SciTools/iris/blob/master/CHANGES, but that doesn't even mention it.
TBH I only put this in because the previous form is actually non-functional -- it is bad Sphinx, presumably just a mistake (unless Sphinx changed).