(This version is tagged 4.0.2, but is the first release in the v4 major version.)
MobX 6 is now supported! This version of MobX replaces MobX 4 and MobX 5, and works in environments that support Proxy as well as those that don't. In the past, MobX 4 was for environments without Proxy, and MobX 5 was for those which supported Proxy. MobX 6 works in both.
Note that MobX-State-Tree v4 will only work with MobX 6, so if you need to stay on MobX 4 or 5, you'll need to also stay on MST 3.
@jamonholmgren is now taking over as lead maintainer. Stay tuned for more announcements and an updated roadmap.