Does this change the location of cached files by default? Or does the user need to opt-in? If they need to opt-in, is that documented anywhere? My preference is to just have good defaults, and I'm okay with improving the defaults.

Yes it changes the location of cached files for P2 and maven users, but not in a lot of user installation.
P2 stores its data under ${user.home}/.m2/repository/dev/equo/p2-data/ which is the default location for maven users.
The proposed improvement is to store P2 data in the configured local maven repository, which is:

• the default location if not configured: ${user.home}/.m2/repository/dev/equo/p2-data/
• the local repository referenced in the settings.xml file under /settings/localRepository element
• the path given on mvn command line with -Dmaven.repo.local=/tmp/repository

The proposed change could also be seen as a fixed because as a maven user you expect downloaded data for build to be stored in the configured local repository.

Into which documentation may I add such change?

Roger. Thanks for explaining! No more docs needed.