Inside the official MODX docs I have found Core folder - Upgrading from 2.x to 3.0 | MODX Documentation
But on several MODX 3.0.2 installations, I moved the core folder outside the webroot without problem.
Is the doc needs to be updated ?
I don’t have the full answer for you, but MODX 3 makes use of Composer, which requires a standard location for the core directory. Any operations that involve Composer are likely to fail with the core moved.