Here are two scholarly articles from the Journal of Food Science Technology:
https://www.ncbi.nlm.nih.gov/pmc/articl ... cle_10.pdf"Recent advances in drying and dehydration of fruits and vegetables: a review"
It says
"The retention of vitamin C in freeze-dried products is significantly higher than that of oven and sun-dried products".
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3722389/"Retention of nutrients in green leafy vegetables on dehydration"
They measured the retention of nutrients in five types of vegetables after oven-drying for 10 to 12 hours at 60 Celsius. They found that
"retention of ascorbic acid was 1–14%, thiamine 22–71%, total carotene 49–73% and β—carotene 20–69% respectively, of their initial content. Dialyzable iron and calcium in the fresh vegetables ranged between 0.21–3.5 mg and 15.36–81.33 mg/100 g respectively, which reduced to 0.05–0.53 mg and 6.94–58.15 mg/100 g on dehydration." Dialyzable means being in a form which can pass through a membrane. So dehydrating does remove most of the Vitamin C (ascorbic acid), and significant amounts of some other vitamins and minerals.
The following article, from the Journal of Molecular Biology...
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5297706/...states that
"during convective drying, the oxidation processes caused by hot air results in significant chemical alterations in the profile and the content of the thermolabile biologically active compounds".
So if you like lots of Vitamin C in your food, go for the freeze-dry rather than the dehydrated vegetables.
All of the papers speak very positively about freeze-drying. It seems the biggest problem is that it's a high-cost way to preserve food. The papers have lots of references, and apart from the Green Leafy Vegetables paper they are literature reviews, which means you will need to go to the references for the original experimental data.