This may seem a dumb question. Has the API for foobar 2000 0.9x changed so much as to not allow backwards compatibility of plugins with <0.83, or is it just a really good and thorough cleanup of the code? A few answers are all that are needed.

That is indeed the reason.

A quick look at the open source SDK would indicate that changes between 0.8 and 0.9 are quite vast and extensive.

Thanks for the speedy reply, I figured as much...