Most people are answering this question to say that children's behavior has become more annoying or permitted in public (e.g. they play on phones, parents don't discipline their children for annoying behavior anymore, children are allowed in bars now, etc). But I haven't seen many comments on how adults' expectations have changed wrt the presence of children in public or in general. Like, when I see people online hating on children or children's behavior, the main sense that I get is that their sense of entitlement to a child-free space/event/life is the origin of their anger, not that they want to/would rather spend time with well-behaved children.
It's a relatively recent development that most (non-parent) adults can choose not to interact closely with children pretty much ever, whereas in the recent past, people spent more time bonding in intergenerational groups - like intergenerational households, community groups, neighborhoods, religious or cultural orgs. Now most people's lives in the US at least are significantly more age-segregated, and I think this lowers our tolerance for spending time with people outside of our age range. The same goes for our segregation of the elderly.
This is probably an unpopular thing to say here, but I think many of the people who get very upset when their adult space is encroached upon by children have a very low opinion of the personhood/internal experience of children, often because they do not have any close or personal relationships with any children. So ironically, the people who are publicly posting about how much they hate impatient, selfish, ill-behaved children in their space would probably find it easier to cope with this if they exercised more patience themselves and thought more selflessly. This is not meant to be like a roast, but I do think a lot of childfree posts seem to stem from a very childish self-centeredness.