I think this paper may be slightly misleading. True, rGFR is currently used in the diagnosis of CKD, but what is eGFR. It is the inverse of creatine levels in the blood. What makes Creatinine? Well Creatin is obtained from eating animal products mainly red meat. I do not subscribe to the creatinine levels being an indicator of ROS or inflammation. It is naturally the overflow from what we essentially need to repair and replace cells in the body. If you do not eat meat, then you still synthesize creatin for that purpose. Only, you will not have an excess so eGFR is higher in vegetarians than in meat eaters or omnivores. The study is very specific in several places to make this point, and I am suspicious of a bias creeping in. They do not test people on iron supplements but assume the source is mainly carnivore which is is not necessarily true.
Again eGFR is used as a marker for kidney failure. But urea is also in the blood, and in kidney failure, this builds up. Lactic acid is another possible marker. I am sure there are other mrkers that the kidneys should remove. After all, it is what they remove in dialysis. So why were they not monitoring these by simple blood test? Because eGFR is king, and yes it too needs to be excreted. But the deck is naturally stacked against meat eaters simply because creatin only occurs in muscle tissue, and not in plants.
The 0.8g protein per kg lean body mass is an upper intake RDA for a reason, it limits the excess to be cleared out as unused. Athletes and body builders can eat larger quantities since they are doing more cell building and replacement, so use more creatine. That is why protein is said to be harmful for the kidneys. Simply because if in excess of what we need, then it stays in the blood until excreted. Actually, autophagia or cell repair occurs at night while we sleep, so blood tests during the day will see an excess of creatinine since we are not using it then.
Having said all that I do think my iron supplement can be reduced for a while, but need to get GP to check for anemia especially if this diet plan reduces ferritin below the recommended lower limit.