It used to be possible to access the unminified versions of (most) Stack Exchange JS files via cdn-dev.sstatic.net (as opposed to the usual cdn.sstatic.net host that serves the minified versions). It was a kind of a "hidden feature", and not very widely documented, although it's mentioned e.g. in this old Blog Overflow post.
For example, the unminified version of https://cdn.sstatic.net/Js/full.en.js used to be available at https://cdn-dev.sstatic.net/Js/full.en.js. For anyone curious, here's an archived copy from 2016 showing what it used to look like.
However, while trying to see if I could help investigate the cause of this bug that I reported earlier today, and maybe suggest a fix for it, I noticed that this no longer seems to be the case: now both hosts serve the same minified code.
Obviously, Stack Exchange isn't under any obligation whatsoever to give its users access to such internal development resources like unminified JS code. If they don't want to let us see them any more, that's their choice to make. But losing this access is definitely going to make it harder for the community to assist SE in diagnosing and fixing bugs, and also makes it a lot harder to develop user scripts and browser extensions that interact with the Stack Exchange UI. Personally, I'm hoping this is just an unintentional glitch, and that Stack Exchange will continue to provide us access to these files. But if it's intentional, it would at least be nice to know that.