Lots of things were disallowed in 0.19, but probably the most disruptive were that custom native modules were disallowed (which basically means that, only certain official packages would now be allowed to directly call native JavaScript), and the package manager was locked down (which means that you can only install packages from the official elm repository, not GitHub or anywhere else).

You can still indirectly call native JavaScript, in a message-passing kind of way (via Ports or custom elements) but these changes were still really disruptive to many codebases.