I see. The extension I installed to test that actually is signed, though it's not in AMO.

I don't like this. I know there have been issues with malicious extensions, so it makes sense to me that installing unsigned extensions is turned off by default, but requiring developer builds is a step too far.

What release made this change effective?

Correct. It's incredibly how much misinformation there is about signing, even here where people should know better. It's very tiresome.