I added `org-summary-todo' example function, that are advertized in
`org-after-todo-statistics-hook', to my init file (with
lexical-binding: t) and got a compilation warning:

  Warning (bytecomp): Unused lexical variable `org-log-states'

since this variable is undefined. The command:

  git log -G 'org-log-states[^-]' --patch -- lisp/

shows its first appearance in commit
2c0812caf14e674d938d80d05678576c0cefcc83 but only in docstring.

Running ~find~ also didn't show its definition:

  find -H . -type f -exec grep --color=auto -i -nH --null \
    -e org-log-states\[\^-\] \{\} +

But there are =org-log-states-order-reversed=, but it doesn't seem to
be relevant.

PS: I didn't add TINYCHANGE since I have reached the contribution
limit but FSF didn't respond yet.