I was born in 1973, so 70s.
Never did sleep overs at primary school. TBH I can count on my fingers the number of times I slept at a friends house.
I do remember birthday parties, but certainly not everyone had one, and most werenot whole class affairs - so maybe 3 or 4 a year??? Possible more.
And play dates? I don't really remember anything organised after school. We used to play with friends who lived on the same stree, who we knew from there rather than school - some would be older, some younger. I wasn't allowed to play on the street when at first school, so we would call for one another and play in the garden. We weren't allowed inside to play much unless it was very wet!