Café etiquette threads never seem to go well in MN.
Round my way - as I think I've said upthread - older people in particular will tend to offer up spare seats when they see people in need. I can't recall many instances of people having to ask.
Maybe there are different attitudes in different generations and different generations.
Many years ago, I spent a term in Leningrad (as it then was) while I was a university student. My birthday landed very early in my stay. I decided to treat myself by having lunch out in Nevsky Prospekt.
There was a queue - hardly unusual in the Soviet Union. The Head Waiter came out and said that there was a place for one...so I got in.
I was discomfited to find that I was being led to a table with a family of five - Mum, Dad and three children. The Head Waiter made it clear that no one had any choice in the matter and I was worried about intruding - they were halfway through lunch.
God bless them, the parents welcomed me with a smile. I thanked them and the father insisted that I have some of the blackcurrant juice that they'd purchased.
They were Azeris from Baku and seemed delighted to be sharing their table with a hapless student from Scotland. I recall that they said that I really should visit Baku some day. They were lovely folk.
I doubt that the same would happen in modern day Petersburg, but for me it was an awkward situation that turned out well.