If the provider specifically documents the condition as iron deficiency anemia due to bariatric surgery status, I would consider D50.8 (Other iron deficiency anemias). Index: Anemia - iron deficiency - specified type NEC
I would also assign Z98.84 (Bariatric surgery status) to help tell the complete clinical story and establish the relationship between the patient's surgical history and the anemia.
I did look at the K95.- category for complications of bariatric procedures, but my interpretation is that those codes seem generally intended for more direct postoperative complications. Given how common iron deficiency anemia can be following bariatric surgery, I'm not sure I would automatically view it as a reportable surgical complication requiring a K95.- code. That's why I'm leaning towards the Z status code.
That's just my interpretation based on what I could find right now. I didn't catch any official guidance, such as coding clinics, in my quick look,
However, maybe others would arrive at a different conclusion or be aware of an official reference I didn't see.