A probably little surprising improvement in MobX 6 is that it supports more JavaScript engines than MobX 5. MobX 5 required proxy support, making MobX unsuitable for Internet Explorer or React Native (depending on the engine). For this reason MobX 4 was still actively maintained. However, MobX 6 replaces both at once.
By default MobX 6 will still require Proxies, but it is possible to opt-out from Proxy usage in case you need to support older engines. And, as a result, it is now possible for MobX 6 to warn in development mode when features that would require proxies are used
🤔