haven't heard or read anything about warfarin and rate of decline in GFR for CKD patients. there's no indication to anticoagulate people with CKD and no other reason.
however, for AF patients with CKD, they are categorically undertreated as a group because of a fear of increased risk of bleeding. while that risk is real, the increase in thromboembolic risk is even greater - the next iteration of the risk score for AF Chads/Chads-vasc will almost certainly have an "r" in it for renal failure. thus these patients need anticoagulation. Depending on GFR this should be a NOAC where possible - their greatest benefit over warfarin is actually a reduction in bleeding, not a huge gain in efficacy (NNT for apixaban over warfarin to prevent a stroke is around 200). The reduction in bleeding is most pronounced in people with reduced GFR (NNH compared to warfarin falls to around 30 if i recall correctly).
so these folks need anticoagulation, and NOACs trump warfarin (but CAREFUL attention must be paid to dosing and once GFR goes below 15 NOACs are out).