The previous version of Elm, 0.18, had a time travel debugger and it was wonderful. Version 0.19 broke something and it no longer works unfortunately. https://elm-lang.org.
There is a PR for that already, but it has not yet been merged: https://github.com/elm/compiler/pull/1850
At CurrySoftware we run a fork of the Elm compiler which merges this PR and has a feature to use ports and the debugger with `elm reactor`: https://github.com/CurrySoftware/compiler
And an example how to use it here: https://github.com/CurrySoftware/elm-reactor-example
The elm-dev channel contains most of the info coming from Evan :). I had the same feeling as you until I discover this.
Meanwhile, there is a forked version of the compiler with a hotfix for the debugger that works for most applications, even medium and large ones: https://github.com/elm/compiler/pull/1850