Allow AddToCartFormatter to hide variant field if only one variant

Are you not using the "commerce_product_variation_title" widget? There is the following setting:

    $element['hide_single'] = [
      '#type' => 'checkbox',
      '#title' => $this->t("Hide if there's only one product variation"),
      '#default_value' => $this->getSetting('hide_single'),
    ];

Perhaps you could elaborate with screenshots as I'm not 100% sure about the issue reported here...

This is the product type view mode settings:

Ah! I had to edit the Order Item Type form display settings, that was the step I was missing.