It's how I grew up pretty much but not quite as rigidly. I'm now 64.
I never, ever saw my grandma in her nightie, not even when she was in a nursing home, until she became bedbound.
My mum will make tea and breakfast in her dressing gown but is always dressed by about 8am (88). Whilst I wasn't expected to come down dressed on non school days, I was expected to get dressed after breakfast. If we had visitors we were washed and dressed.
I go downstairs in my dressing gown, feed the cats, empty the dishwasher, put on a load of washing, make a second round of tea (DH makes a very early one), and take tea and breakfast back to bed and footle on MNet for 40 minutes. Doing it now. Then I get washed and dressed and start the day. By about 7/7.15 Monday to Friday. When the DC were small, they used to have breakfast in their pyjamas in the family room, and dd always got dressed on the sofa, watching TV. It worked for us and kept their uniforms clean.
The DC tend to get dressed before going downstairs. DS's wife goes down in her Jim jams and dressing gown; I've never seen dd's bf in his pyjamas but he's from a very traditional family.
It's probably generational.