For package maintainers, here's an example of how to adjust to the new interface, since it no longer writes to stdout:

https://github.com/jj-vcs/jj/pull/5258/files#diff-206b9ce276ab5971a2489d75eb1b12999d4bf3843b7988cbe8d687cfde61dea0

Not sure if this is the right place to highlight it, but this broke --HEAD builds in Homebrew:

• https://github.com/Homebrew/homebrew-core/blob/488e84fdb8e9ea309dff4b507e170edd6e92f38e/Formula/j/jj.rb#L46

$ brew install --HEAD jj
==> Fetching jj
==> Downloading https://raw.githubusercontent.com/Homebrew/homebrew-core/2c92fccaa0f234d8d34a2a2e68c12d438b4c42b7/Formula/j/jj.rb
Already downloaded: /Users/zx8/Library/Caches/Homebrew/downloads/629dbff562ae32e7c6f7ef688c739356c1488fc61a597432e69c8b194e268be7--jj.rb
==> Downloading https://github.com/jj-vcs/jj/commit/b4f936ac302ee835aa274e4dd186b436781d5d2f.patch?full_index=1
Already downloaded: /Users/zx8/Library/Caches/Homebrew/downloads/4f0b715f9452bf91255eb771e1f8462f949705770cd700fb93f6b4fdbac4dd65--b4f936ac302ee835aa274e4dd186b436781d5d2f.patch
==> Cloning https://github.com/martinvonz/jj.git
Updating /Users/zx8/Library/Caches/Homebrew/jj--git
==> Checking out branch main
Already on 'main'
Your branch is up to date with 'origin/main'.
HEAD is now at d3f56c0ac git: forward "remote: " lines when they arrive
==> Patching
==> Applying b4f936ac302ee835aa274e4dd186b436781d5d2f.patch
==> cargo install --path=cli
Error: Failure while executing; `/opt/homebrew/Cellar/jj/HEAD-d3f56c0_2/bin/jj util mangen` exited with 2.

— @zx8 in #5258 (comment)

[...] every package maintainer who currently runs jj util mangen will try to upgrade jj, see that their packaging no longer works, and will need to debug.

• Currently, I guess their best option is to trawl the changelog, or even bisect.
• Unlike most commands: I suspect that deprecating the command probably won't help, because the deprecation notice will probably be silently swallowed? This is in contrast to commands for interactive human use.
• So the best option to inform package maintainers is probably to make the command produce a hard error, but with a helpful message to tell them how to fix it?

We should see if we can do this before the next release to make it easier for package maintainers to upgrade.

— @arxanas in #5258 (comment)