Thread:User talk:CodeCat/Template:categorize not categorizing "Category talk" pages/reply (2)

Was that discussed somewhere?

is meant for many namespaces. If it's true that is meant for entries only, then  should not use.

But rather, I'm skeptical of your statement above. I believe is good for many namespaces, and the fact that it doesn't work in "Category talk" is a bug to be fixed.