Every single version of Elm has had breaking changes. However, 0.17 -> 0.18 had an update tool that automatically fixed most of those changes, and a tool for that will be available for 0.19 as well.

NoRedInk (where Feldman works) are already updating a bunch of code to 0.19, according to Luke's talk at Elm Europe 2018, so they seem pretty sure of where things are going. Evan doesn't like announcing things until they are done though.