Too late, I'm afraid. That horse has literally* bolted. The world has decided on what "steep" means now.

* http://www.salon.com/2013/08/22/according_to_the_dictionary_...

Ok, so let me make it graphical for you: If the 'Y' axis of the graph is expenditure of energy or time and the 'X' axis is progress in understanding then a steep curve indicates a lot of expenditure for relatively little progress.

That's the 'informal use', as in, everyday communications such as, but not limited to comments in online communities.

That is not a learning curve though...

The "informal" usage stems from people not knowing what they are talking about. No need to perpetuate it if you know better.