I have been here rather longer than you, and not noticed a decline in the time period you’ve identified. I do note however there seems to be a tendency in all communities for people to think it declined since they started using it.
A good example is this: https://news.ycombinator.com/item?id=19140862
I don't remember seeing that kind of politics on HN in the past.
Anyway, I don't really want to include a value judgment here; I'm just observing that it changed.