Where can I download the C++ Photoshop Plugin SDK?

The Adobe Photoshop plug-in SDK for C++ is still available! You'll need an Adobe Developer account to login and agree to Adobe's license terms. The file you're looking for is now called the "Photoshop Plug-In and Connection SDK" and is available from this link at developer.adobe.com:

https://developer.adobe.com/console/servicesandapis

Since the download can be hard to find, this blog post has screenshots that will help you find it:

https://blog.developer.adobe.com/photoshop-plugin-support-for-apple-silicon-required-recompile-for-c-plugins-efadd3ea0096

If you're downloading the Photoshop plugin SDK for use with recompiling FilterMeister, the exact SDK version shouldn't matter, as it hasn't changed very much since 1999. However, I'd still recommend downloading all the SDK versions for safekeeping. Adobe has removed downloads in the past, so keeping old versions in your own archives is a good idea.