lisp/startup.el から:

  (unless noninteractive
    ;; DOS/Windows systems have a PC-type keyboard which has both
    ;; <delete> and <backspace> keys.
    (when (or (memq system-type '(ms-dos windows-nt))
              (and (memq window-system '(x))
                   (fboundp 'x-backspace-delete-keys-p)
                  (x-backspace-delete-keys-p))
              ;; If the terminal Emacs is running on has erase char
              ;; set to ^H, use the Backspace key for deleting
              ;; backward and, and the Delete key for deleting forward.
              (and (null window-system)
                   (eq tty-erase-char 8)))
      (setq-default normal-erase-is-backspace t)
      (normal-erase-is-backspace-mode 1)))

x-backspace-delete-keys-p is a built-in function.
(x-backspace-delete-keys-p &optional FRAME)

Check if both Backspace and Delete keys are on the keyboard of FRAME.
FRAME nil means use the selected frame.
Value is t if we know that both keys are present, and are mapped to the usual X keysyms.

これが問題らしい。nil になってしまう。