It is well established in the scientific literature that those cooking on stainless steel cookware get a portion of their daily iron intake from the iron in the pan that makes it into the food:   Geerligs, Brabin, and Omari, Food prepared in iron cooking pots as an intervention for reducing iron deficiency anaemia in developing countries: a systematic review, J Hum Nutr Diet. Aug 2003, Vol. 16, Num. 4, pp. 275-81. Kollipara and Brittin, Increased iron content of some Indian foods due to cookware, American Dietetic Association. Journal of the American Dietetic Association, Vol. 96, Num. 5 (May 1996) Kuligowski and Halperin, Stainless steel cookware as a significant source of nickel, chromium, and iron, Arch. Environ. Contam. Toxicol. (1992) Vol. 23, Num. 211.   For most people that's not a bad additional nutrient, and those who saw family use a stainless steel pan surviving a lifetime know that the lost steel will not wear a pan out, not even after decades of use.  I'm now giving up on finding a tri-ply skillet whose bottom will remain perfectly flat. Since I cook on ceramic, a cast iron skillet is a good option. I'm hoping this would lead to steaks cooked perfectly evenly.  Suppose I'll use the cast iron skillet for literally just steaks, I'm concerned about the burned fat from the skillet's seasoning that will leach out to the steaks—in analogy with what happens when cooking in stainless steel cookware.  When you cook with cast iron, how do you know that some of the fat from the food you're cooking makes it to the skillet, but never the other way around. How do you know that the burnt pan "seasoning" does not make it into your food? Without some kind of confirmation that the burnt pan seasoning is safe, it would be nice to confirm that this does not occur in even trace amounts?  I'm guessing that deglazing the pan after cooking steaks would only increase the chance of this leaching in the undesirable direction. Hints for safe deglazing are also welcome.  update: A possible litmus test to determine potential leaching is to wipe with a paper towel. 
You don't know it. In fact, I think that trace amounts of it end up in the food. With very new seasoning or also badly made seasoning, I have seen some "wear out" on pebbly pans. Not outright flaking, simply the hills of the pebbles looking a different, unseasoned color after dryish cooking on higher temperatures.   The sources you read are a further hint. The one you linked is the only one I've ever seen claiming that stainless steel provides dietary iron, and it is written confusingly, starting to talk about SS but later saying that it is iron pans which contribute to dietary iron. There is some exchange in matter between the pan itself (beneath the seasoning) and your food, and as far as I am aware, also between the seasoning and your food.   Third, the above is about standard, well made seasoning, which still gets somewhat abraded and repaired in microscopic amounts with each cooking. If you manage to burn stuff onto seasoning (carbohydrates in combination with high temperature are the worst there), it won't clean completely with scrubbing, but will continue coming of with use. No visible residue in the food, but months later, the rough charcoal structure will be gradually gone.   I don't have any firm sources from somebody who measured this in a lab. But from everything I have observed while using and misusing seasoned pans, there is some transfer happening in both directions, in trace amounts.  