Vue 3 core has been in beta for over two months now. We have merged all planned breaking change RFCs, and there are no further breaking changes planned before official 3.0 release.
Thousands of early adopters have been using it for new projects and have helped us identify and fix many bugs and behavior inconsistencies with Vue 2. At this stage, we believe Vue 3 core is quite stable and ready for RC.
The only difference between Vuex 4.0 and 3.x is that it's Vue 3 compatible! It is ready to enter RC together with Vue 3 core.