If you have e-commerce site or store I advise you not to delete them. Because we don't know how many places we shared the URL and build backlinks for the specific URL.
So if you delete it, users will get 404 or if you redirect the URL it will leave the bad impact so Simply You can put OUT OF STOCK label.
And if possible show the related products so that user gets what he or she is looking for.
I hope this helps.
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