its in the beis yosef on the tur but not the shulchan aruch (which was written later). the kabbalists give it credence.
funny that the best known ashkenzi dish, bagel with cream cheese and lox, is deemed unfit by these sources.
some sefaradim do not eat it based on this beis yosef and the ben ish chai’s kabbalistic analysis of same. most folks do not live by the kabbala but by halacha. since its halachic origin is murky (some reputable poskim hold its a TYPO in the tur and it should have read meat and fish) they eat it.