Any deterioration should be addressed. My eGFR was stable in the 50's for a long time, then dropped to 36 suddenly, so I then saw a nephrologist. But a stable 3GFR in the mid-50's or even low '50's didn't worry me that much and my doctors never mentioned it.
I have serious pain and was grateful to continue with my strong NSAID, sparingly, for a few more years. Noone told me to stop and noone thinks the drop in eGFR is from meds (I have lupus). But if I had had a more gradual drop, I would have totally eliminated NSAID's earlier. And taken the drop very seriously.
As for diet, my nephrologist recommendations are to push salt and protein, the opposite of what most kidney diets say. I needed to be bad off enough to get thorough testing (ultrasound, autoimmune testing, echocardiogram) to see if low blood pressure or low heart ejection fraction was contributing to a lower rate of filtration in the kidneys.
Muscle loss is another factor in the eGFR along with hydration, of course, so I get a Cysstatin C which is more independent of those factors.
I'm curious as to why your nephrologist pushes salt and protein. What's her reasoning since, like you say, it goes against conventional recommendations? I currently try to limit both.