Variations are now managed on their own tab

Product variations used to be managed directly on the product add/edit form, using the "Inline Entity Form (Complex)" widget.
Since Commerce 2.10, variations are now managed on a separate "Variations" tab. This will allow extending the listing with bulk variation generation, and bulk updates (replace image, change price, etc) in future releases.

Sites that wish to revert back to the previous UI can modify their product type's form display (e.g. "admin/commerce/config/product-types/default/edit/form-display" for the "default" product type) and re-enable the Variations field with the "Inline Entity Form (Complex)" widget.
They can then implement hook_local_tasks_alter() to hide the Variations tab:

function yourmodule_local_tasks_alter(&$local_tasks) {
  unset($local_tasks['entity.commerce_product_variation.collection']);
}