We�d say that it�s incorrect that ENIG solderability protection cannot be steam aged. It�s true the immersion tin and silver cannot be steam aged.
We believe that most of the issues surrounding the poor performance of ENIG after steam aging is attributable to inadequate gold plating thickness. Thin gold plating allows corrosion of the nickel undercoat. Corroded nickel is very difficult to solder.
There�s a fair amount about this on the web, for instance: http://www.npl.co.uk/ei/presentations/storage.pdf
It�s possible that IPC-4552, Electroless Nickel/Immersion Gold (ENIG) Plating for Electronic Interconnections addresses your concerns.
On the shelf life concern, many have posted comparisons of various solderability protections on the web. Here's one example http://www.twi.co.uk/j32k/unprotected/pdfs/6feb03_aw6.pdf
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