"SO Proper" is typically said in the following contexts:
A user asking a question in chat that should probably be composed and asked on Stack Overflow, as that's the 'proper' place to ask non-trivial questions. E.g. "You should ask that on SO proper ..."
A user asking a similar question as a comment, or an answer to an existing question, though here it's much more common to just say 'proper', or 'properly'.
Asking for reference material that would be desirable on Stack Overflow if asked as a 'proper' canonical question and answer. E.g. "That blog post should be on SO proper!"
In your example, a few words were left out of the title. It was asking if Stack Overflow was a proper place for something, not really in the context of "SO Proper". However, yes, it is a term that is used semi frequently.