Debating historical design decisions that effectively cannot be changed is off topic for the tracker (but fair game on python-list).  However, I will explain this much.  The s.isxyz questions are answered by examining the characters and codepoints in s.  Answering iskeyword(s) requires reference to the collection of keywords, kwlist, which must be exposed to users, and which may change in any new version.  We usually attach constant data attributes to modules (other than builtins), and never (that I can think of) to built-in classes.  "def iskeyword(s): return s in kwlist:" belongs in the module with kwlist.