There are less exclusions happening in schools, because schools are now being judged on how many they exclude. The behaviour has not improved, it's just that schools have stopped excluding anyone, whether they need to be excluded or not. Exclusion rates are not an indication of what the behaviour is like in schools: observing behaviour in schools properly is.
I can't really add anything to this topic that Tom Bennett hasn't said, so I think you should just go and read his post about it. It's brilliant.
I will say that less exclusions only undermines teachers further, takes more power away from them, and makes their job much more dangerous, both physically and to their mental health.