On top of the already-mentioned JS interop breakage, Elm 0.19 also dropped native Websocket support[0]. The API had issues (fair), so it was dropped rather than improving it due to wanting to do it perfectly (okay, I guess), buuut due to the JS interop restrictions this meant that 3rd-party experiments or alternatives were impossible (??), which meant that any use of Websockets was in practice now completely impossible! If I recall correctly something similar happened to other core libraries.

This "nobody is allowed to do this until Evan himself has made time to come up with a blessed solution" style of development left a lot of people quite disappointed. Elm was marketed quite heavily as the best thing since sliced bread and the future of front-end web development, but in reality it turned out to be just Evan's toy language which you could look at but weren't allowed to touch. Which is of course allowed, but it does rapidly kill any kind of community around it.

[0]: https://github.com/elm-lang/websocket

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.

One form of JavaScript interop. Instead of being able to write bindings directly to native code, you had to pass messages through "ports" instead.

https://discourse.elm-lang.org/t/native-code-in-0-19/826

Personally, I was sad to see signals and FRP go in 0.17

https://elm-lang.org/news/farewell-to-frp