Yes, of course, Shopify is the best platform for ecommerce. when it comes to the robots.txt, we can't modify anything for shopify. because shopify is already optimized with best possible way based on the search engines. It's is automatically generated by Shopify for every website. You don't bother about it, Google makes it's optimal and your page extensions are indexed properly. so don't need to make any changes on the robots.txt file for shopify.
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