Honestly, the very best information and support IME, is available from the specialized online community that's been running for the last 20 years - their 'home base'/website is the one I gave you there. How their group 'works' is covered here: Tanya's Support Group. The group itself is here: tanyackd - groups.io
The company who makes Kidney Support Gold makes a huge number of holistic supplements for dogs and cats....all fairly high priced. Basically, they are a combination of the plant based products (for the most part) that are touted on any alternative health website which purports to cure many serious diseases. Many of their ingredients repeat from product to product running the usual gamut...astralagus, mushroom supplements, etc. I have never used any for any animal although they may be valid as supplements to standard medical care. The products they sell have no ability to treat or cure directly.
I am not saying that the product would not help to support your cat but I would not replace it with standard medical advice and would ask the vet about its use, especially with interactions with any real medications.
I've had three kidney cats, and tried to give them some kidney support supplements (not that one specifically), but the problem with kidney cats is that they quickly lose their appetites, so I could never get any of them to take any supplements mixed into their foods, and and didn't want to stress them by giving them pills. So...since most supplements that I found at the time were quite large capsules, I just gave up trying anything except prescription meds which I could get compounded into transdermal gels or injections.
Anyway, I wholeheartedly agree that you should check out the above mentioned website. It was my "go to" when my kidney babies were still with me. Has such great information in it, including info about all kinds of supplements, as well as prescription medications, techniques for helping them with nausea, getting them to eat, etc. Unless you join the group though, they are not interactive.