Is Stack Overflow Accessible in China?

Yes, I believe it is. Looking at a survey from Stackoverflow. China actually has a good amount developers access stack overflow. As far as I know. people use VPN to access it whenever possible. https://insights.stackoverflow.com/survey/2018/

there's also a similar Chinese stack overflow which also has a very good reputation among Chinese developers and it's been widely used. It's called 思否（sifou） http://segmentfault.com/