This was originally going to be a response to Rosina1871, but I then decided to make it a separate post as I would like to see other people's comments on my own experience.
I too understood that 'a low carb diet is necessary when on steroids to avoid weight gain', so for a time I diligently followed the low carb way of life. I did lose some weight (though it plateaued) but felt increasingly unwell and became nauseated firstly by cheese, then by fish, eggs and meat. Eventually last December I adopted a vegan diet, assuming I'd gain weight but maybe feel better. Since then I've lost more weight, slowly but steadily at about 0.5-1 lb per week, and more importantly I've lost the constant sick feeling. My HbA1c level is 32.
In some important ways my new diet - commonly described as 'low fat plant-based' - is similar to the low carb one in that I still avoid sugar and simple 'white' carbs and processed foods and I eat plenty of vegetables, salads and fruit. However there's no doubt that with all the grains and pulses, I take in far more carbs and being oil-free I have a low fat intake, so for me at least the orthodox advice evidently doesn't apply. I was already on a low-ish dose of pred when I started, 6.5 mg in December, 5.5 mg now, so I can't say how it would have worked when I was on a high dose.
One other advantage is that I'm now spending far less money on food. Obviously a vegan diet that includes a lot of plant-based processed food can be expensive, but I eat very few of those things. Plant-based milks can also be expensive, but I've found that Aldi's soya milk, oat milk and almond milk are fine and less then half the price of the branded ones.