Accepted answer
Six sessions a week against 800 kcal a day — 5600 kcal for the week — and at that intake the binding constraint is almost never the session count. Protein at 4 kcal per gram is the reason: 100 g a day is 400 kcal, 50 per cent of 800, and 2800 kcal of the 5600 for the week spoken for before anything else is eaten. 6 sessions also has to be recovered from on 800 kcal, and recovery is the first thing an energy deficit takes. Load matters more than frequency here: the stimulus that preserves mass is mechanical tension, so keep the weight and cut the volume rather than the reverse. None of this is medical or dietary advice.
Answer first: the objective in a deficit is retention, not growth, and retention needs load maintenance and adequate volume rather than progression.
Beginners retain the ability to gain muscle in a moderate deficit for several months; trained individuals should expect maintenance and be satisfied with it.
Load is the retention signal. Reducing weight and adding repetitions preserves the workout and weakens the stimulus, which is exactly backwards for this objective.
Training-to-failure comparisons show similar hypertrophy with substantially greater fatigue when sets are taken to failure rather than near it.
Ten sets per muscle group per week, twice weekly, loads held. That is most of it.