Bodyweight squats, exercises from the 'Knees Over Toes' program, single-leg pistol squats, and pull-ups are sufficient to maintain muscle mass for a 5'9" individual weighing 165-170 lbs, even without barbell training.

“I'll do body weight squats in the morning I'll do some of like Ben Patrick's knees over toes stuff I'll do some single leg I love Single Leg pistol squats um and I'll do pull-ups throughout the week but I don't do much beyond that I'm not really even picking up a barbell most weeks and my muscle mass stays pretty constant”