What magic is saying "obsolete variable"

[Lennart Borgman]

I am trying to understand how to get rid of the "obsolete variable"
warnings (and taking care of them, of course), but there is some magic
going on that I do not understand

When stepping through this in edebug

      (mumamo-save-buffer-state nil
        (remove-list-of-text-properties (point-min) (point-max)
                                        (list 'fontified)))

I get the warning below after passing the next last `)':

  Warning: `font-lock-beginning-of-syntax-function' is an obsolete variable (as
    of Emacs 23.3); use `syntax-begin-function' instead.

The macro `mumamo-save-buffer-state' is just a copy of
save-buffer-state in font-lock.el (which is not exposed by
font-lock.el).

Can anyone please explain why I get the warning here and maybe what I
can do about it?