> Do you have a source that 40% muscle loss is typical for a caloric restriction diet without GLP1 agonists?

OK I actually checked up on this, and it is more like 30%, but that number gets worse as you get older. For young healthy men it can be 20%, but as you get older that number gets worse and worse.

I'd want to see a comparison of a similar cohort of people going on a calorie restricted diet of the same magnitude, with a similar (lack of) activity levels.

The study at https://www.thelancet.com/journals/landia/article/PIIS2213-8... compares people who had less overall weight loss on a pure calorie restricting diet, which, well, by definition isn't the same thing.