There are two completely separate angles of supporting kidney health that prescription foods tackle.
1)
They are low in phosphorus/ phosphate. The kidneys have to work quite hard in removing this from the bloodstream, and so a reduction in this mineral in the body eases their workload.
You can support them in this way either with foods with a low phosphorus level (usually prescription renal foods) or with Ipakatine. Ipakatine is a phosphate-binder so it essentially takes some of the strain for doing that job off the kidneys. As
@lullabydream says since you are using Ipakatine, there's no need to also feed the prescription food. Phosphorus is a mineral that is essential for life, so you're only aiming to reduce the amount in the body to support the ageing kidneys, not remove it entirely. If and when Tilly's kidneys further decline, you can increase the Ipakatine dosage independent of everything else - which gives you more control for balancing her health needs.
However also
2)
They contain hydrolysed protein. That is protein which has been chopped up into smaller strands. Which means the kidneys don't need to do as much of the chopping up work themselves. Different proteins require the kidneys to do more or less chopping up of protein during digestion. The more chopping up required, the harder they have to work, the faster ageing decline will be.
This means that if you're going to switch off the prescription renal food, you need to look at other foods which don't contain ingredients which need a lot of protein-chopping, or, in more technical terms, are easier to absorb and process.
Proteins which support the kidneys in being easier to process: fish, egg, poultry, barley, maize, rice, potato
Proteins which will speed up kidney decline as are harder to process: red meat (especially beef and venison), legumes (peas, soya, beans), oats, wheat.
So yes you can safely switch off the prescription renal food if you keep up with the Ipakitine, but whether or not it's appropriate to go back to what you were feeding before depends on further ingredient details.
If you share what it was she was eating before, I can take a look at the nutrition details for you,
@Colliebarmy.
As an aside, you can see that the details are kind of complicated and especially difficult to explain to an owner who likely isn't going to take everything in during a short vet consult window, so vets will often take the route which leads to their patient definitely getting support rather than fully explaining it all and crucial parts being missed. Your vet should be supportive if you explain all the details of your understanding to them going forwards though
