Re Blender plugins…
On Blender Market and Gumroad a lot of the listings for Blender plugins don’t seem to obviously specify Windows/Mac/Linux, so I was wondering if there is some magic Blender language where I can just assume they are just cross compatible, or if I need to search for the small print… Many thanks. 
Cross-compatible unless it says otherwise.
It’s tends to be python, thus, yes, as you surmise.
Are there any “plug-ins” that are not “Pure Python?” I really don’t know the answer to that, and so I am curious.
Nope- anything beyond that requires re-compiling Blender
Incorrect. The Python interpreter, and therefore any addons, can interact with code compiled in other languages without touching Blender source code. It’s also quite common for addons to rely on binaries in a manner where the addon is just an interface.
For example there is a SketchUp importer which uses the SketchUp-SDK… available for Windows (Mac?) but not for linux.
Yes, I know that Python can load and interact with a binary library without involving Blender. (Some “built-in” language features are done in just this way, as is true of most languages.) And yes, I can very easily see how an importer/exporter would do just that – leveraging the vendor’s own public SDK/API to “future-proof” the handling of the files.
I really was just idly wondering if the practice was widespread … where the core logic was done in this way for reasons other than compatibility.