There are ways to debug minified JS in production, sourcemaps being one of them.
In addition to the speed difference, there's a significant payload size difference: something like 1.5+MB for the dev build, vs 70kb for the production build, last time I checked.
Sourcemaps have the drawback that you actually need to generate them first. Sometimes, getting all of the tooling together is teedious.
And about the size: if the JS file is cached properly, downloading 1.5mb once doesn't seem like a huge issue, especially for something like Slack where only desktops use the web client (most mobiles probably download the Slack app). I don't know if this was the case here though.
In addition to the speed difference, there's a significant payload size difference: something like 1.5+MB for the dev build, vs 70kb for the production build, last time I checked.