>> (setq tab-line-format "xxx")
>> (global-tab-line-mode 1)
>>
>> value "xxx"  will lost.
>
> Yes, tab-line-mode resets the variable unconditionally:
>
> (define-minor-mode tab-line-mode
>   "Toggle display of tab line in the windows displaying the current buffer."
>   :lighter nil
>   (setq tab-line-format (when tab-line-mode '(:eval (tab-line-format)))))
>
> Perhaps that should be done only when the variable's value is nil?

This needs also to keep the original value while disabling the mode.
Do you think this should be installed in Emacs 28?