Where can I download the Photoshop plugin SDK?

I've been looking for the Photoshop plugin SDK for Windows (for making things like 8BF filter files, and 8BI image loading plugins, etc), but Adobe seems to have hidden those SDKs outside of Google's ability to find them with its web crawlers. I can't find any link via Google that even gets me to an Adobe downloads page that would let me download the SDK. The closest thing I get is the webpage for the Photoshop CS plugins SDK, which requires I do stuff in Javascript (not C++ like standard Photoshop plugins), and it seems to not be so much a Photoshop plugin as it is an Adobe ecosystem plugin (even requiring me to have an API key to make a plugin). It seems to be more of a "connected" thing (interfaces with the internet, more than interfacing with Photoshop). Even the file extension isn't the same anymore. Now it seems to be UXP too. I know for a fact this has nothing to do with classic Photoshop. This is some new thing for Photoshop CS, which I don't have. I have quite an older version of standard Photoshop, so all this UXP stuff is useless to me. Where can I download one of the old legacy Photoshop SDKs? I ask this here, because it seems Google can't find it.