As a developer or security researcher, you're able to download and run GitHub Enterprise Server. I'm not sure having access to the full source code makes a meaningful difference for most of GitHub's surface area, given it's largely Ruby.
describe is also the command you can use to edit the commit message of the change you're currently drafting. In jj there's no staging area, every modification to the working tree immediately gets integrated into the current commit. (This means if you have no diff in your working tree, you're actually on an empty commit.)
It was so much of a problem that at work we added a check that you were charging from the right ports to our internal doctor script (think like `brew doctor`).
I presume that would unfortunately break websites in _other_ ways, if they're UA-sniffing Firefox in unconventional ways. At that point they might as well remove it altogether instead of drastically changing the UA string.
Mozilla froze the macOS version in User-Agent strings past Catalina too, also due to web compatibility issues (particularly with older versions of the Unity game engine).