From that utterly unrelated paper on the beloved topic of adaptive thermogenesis aka starvation mode, subjects preserved ~90% lean tissue at ~10% total mass loss (highly targeted nutrition + exercise).
As for a more pessimistic expectation from someone who was utterly sedentary, didn’t care much about protein and lost 35% total mass (initial BMI = 30 @ 178 cm (5 ft 10 in); current = 20; initial body fat = 30%; current = 15%), around 40-50% of that was combined water + muscle (muscle itself is ~70% water).