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Ideally, should a diabetic cat be kept indoors?

 

I say yes since then you can monitor their eating and vomiting and other things yo can do for an indoor-outdoor cat

 

It would be exceptionally difficult to manage a diabetic cat on insulin when you could not be certain if the cat would show up for shots, nor if there were access to high carb foods where the cat roamed.
There are some finagles you might consider, such as constructing a screen enclosed 'catio' where the roaming was somewhat limited to that area and grass, sun, shade, shelter, food, and water were available to the cat while in the catio.