I'd draw a distinction between toddler behaviour from the under 3s and behaviour from older children.
Toddlers are prone to tantrums and often will have them, regardless of parents. I guess people used to be less willing to let them happen in public places and removed their children quickly if they started to tantrum, or were less likely to take that age child into restaurants etc because they felt such behaviour wasn't appropriate for a restuarant...but today, lots if parents see tantrums as normal (which they are) but also have decided that their day to day life shouldn't be impeded by being in a tantrum phase - so lots will think they are perfectly entitled to be in a cafe or restaurant or supermarket and to let a tantruminf child carry on for 30 minutes, because they don't want to leave. I think there is less consideration of others and the impact of children's behaviour on others and a sense of entitlement to do whatever you like and that everyone has to just put up with whatever children are doing. The behaviour might not be different, but parents attitude towards it is and their willingness to let it happen in public or inflict it on others has changed.
Once children are beyond the toddler tantruminf phase, I think it is possible to see changes in behaviour, and that much of that is to do with parenting. I agree that less smacking goes on, which is a good thing, but also that alternative behaviour management approaches haven't really replaced it. Many parents are very passive and allow children to choose their own behaviour even when very small and even when it impacts on others. They are often reluctant to use any form of discipline when in public because they use it rarely and when they do, get a bad reaction from the children and do t want to go through that in public. But that's the thing isn't it....it's all about consistency and consistency requires effort over a sustained period. When it is applied consistently, children learn what is and isn't acceptable and when it is applied in public they will then respond. However if it isn't usually applied and an attempt is made to apply it occasionally in public, not surprisingly it doesn't work.
Teaching children appropriate behaviour is hard work. It takes a long time and consistency from parents and across situations. Essentially, I think many parents can't be bothered to put in the hard graft that is required to make it work. Inconsistent attempts don't work but just cause conflict which parents find embarrassing in public, so decide it's easier to just let children mostly do what they want and largely ignore poor behaviour. And over time, when you ignore it enough, you probably forget the behaviour is poor as it becomes normal, and when you see good behaviour it is surprising and unusual, rather than the norm.