> Unless the benevolent dictator pushes out a new Elm version that is not backward compatible.

That wouldn't break your build/deploy out of the blue, as you don't get force updated to the latest version.

And having worked on the 0.19 transition, it was in public preview for about 3/4 of a year and regularly discussed in the elm slack. When 0.19.1 came about, there were some more pre-release versions, where evan asked people to test and give feedback in slack. Can't speak for versions before that.