(with-eval-after-load 'notmuch
  (defun my-notmuch-search-show-read ()
    "Suppress `unread' tag filtering in `notmuch-search'."
    (interactive)
    (when (boundp 'my-notmuch-search-showing-unread)
      (user-error "Already showing read messages"))
    (let ((buffer-name (concat (buffer-name) "/ALL")))
      (if-let* ((buffer (get-buffer buffer-name)))
          (switch-to-buffer buffer)
        (progn
          (notmuch-search-edit-search
           (string-replace (format "%s" '(tag unread))
                           (format "%s" '(or (tag read) (not (tag read))))
                           (notmuch-search-get-query)))
          (rename-buffer buffer-name)
          (setq-local my-notmuch-search-showing-unread t))))))

(with-eval-after-load 'notmuch
  (keymap-set notmuch-search-mode-map
              "TAB"
              #'my-notmuch-search-show-read))