Shopify actually won't allow you to create a 301 redirect under the /collections URL, and I think maybe /products though I don't recall exactly.
You can create them, but they don't work, unless this is something that has since been changed. Last time I questioned it, it was apparently working as designed.
If you delete any products that are getting traffic or search engine rankings then you will eventually lose that traffic, especially if you don't have a 404 page setup. You should 301 redirect any deleted products to the most appropriate live product or category page.
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