The vet should advise on this and a good vet won't let an animal suffer. Nor should an owner. We lost our beloved cat at age 14, I thought would get another year with him so it was a shock. Presumably your cats op didn't go well for the kidneys to suffer further - or maybe it was a delayed...
Just done a bit more research, sisal is made from agave and the itching can be due to a chemical calcium oxalate. This irritant is a way of the plant excreting too much calcium taken up from the soil, oxalate-type chemical is used in cleaning products, can get rust out of fabrics... so it's not...
Just got new scratching post..instant allergy, awful itching throat, scalp. Will try hoovering it and hope it dies down. No problem with the last one which was old and inherited. Last time had these symptoms was in a barn full of hay, it doesn't feel like a mould allergy. Bit worried about...