Hmmm. Deprecations are a trick business... There are a couple of helper functions recently added to matplotlib which make the whole process less random - I'm tempted to suggest we take a copy of them... https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/cbook.py#L125

We should also keep track of the version that they were deprecated in.

Finally, it hadn't occured to me that the central_latitude would be set - the need for two keywords makes the whole thing a *little* less desirable, though I still agree that they are not first class projections. @rhattersley - thoughts? There is a similar debate to be had about the OSGB and OSNI "projections"...