>>   (log-edit-mode . ((log-edit-font-lock-gnu-style . t)
>> -                   (log-edit-setup-add-author . t)))
>> +                   (log-edit-setup-add-author . t)
>> +                   (display-fill-column-indicator-column . 78)
>> +                   (eval . (display-fill-column-indicator-mode))))
>
> This will cause an annoying message and prompt when editing Emacs
> sources with an Emacs which doesn't yet have
> display-fill-column-indicator-mode, right?  Can we avoid that?  I
> routinely need to work on the latest sources with an older Emacs.

Shouldn't local-variables functions ignore undefined variables and commands?
Probably not, since such change won't help for older versions.

Then one way is to put such lines to the init file
to avoid typing `y' to confirm local variables
while using emacs-26 to commit emacs-27 changes:

  (put 'display-fill-column-indicator 'safe-local-variable 'booleanp)
  (put 'display-fill-column-indicator-character 'safe-local-variable 'characterp)
  (put 'display-fill-column-indicator-column 'safe-local-variable
       (lambda (value) (or (booleanp value) (integerp value))))
  (defun display-fill-column-indicator-mode ())

And for emacs-27 and future versions this patch is required as well: