Hi Paul,
Just a thought: Have you considered contacting Amazon directly and asking them about this and what to expect in the future?
Given the stats for Amazon Link plug-in use and the fact that it is consistently listed in almost every “great WordPress plugins to use for Amazon” article in the English language, it’s safe to say that your plug-in is driving a bloody massive amount of affiliate sales to Amazon properties. Affiliate sales could fall off a bit if your plug-in stopped working and no further updates were made.
What I’m saying is… you may have a bit of leveraging position to get answers, etc.
Now, I don’t know if this has anything to do with it or not, but I noticed – just looking between Amazon US, Canada, and UK – there seems to be a disparity in what affiliate advertising offerings Amazon itself puts out. For example, the US doesn’t seem to have the sidebar widget anymore, but Canada still has it. The big shiny thing Amazon has rolled out for US affiliate marketing is not currently available for Amazon Canada or UK, as best as I can tell. So I’m wondering if the change may have something to do with changes they are making to their own affiliate marketing tools, as opposed to any specific intent to shut out third party developers.
I say this because Amazon has a vested interest in acquiring sales through any channel legally possible, including third-party plugins and tools.
Again, just food for thought.
Cheers!