x86 is broadly extendable. APX adds a REX2 prefix to address the new registers, and also allows using the EVEX prefix in new ways. And there's new conditional instructions where the encoding wasn't really described on the summary page.

Presumably this is gated behind cpuid and/or model specific registers, so it would tend to not be exposed by virtualization software that doesn't support it. But yeah, if you decode and process instructions, it's more things to understand. That's a cost, but presumably the benefit outweighs the cost, at least in some applications.

It's the same path as any x86 extension. In the beginning only specialty software uses it, at some point libraries that have specialized code paths based on processor featurses will support it, if it works well it becomes standard on new processors, eventually most software requires it. Or it doesn't work out and it gets dropped from future processors.