This transition is from git-annex sync needing --content to send content to repositories that have preferred content configured, to sending content by default to such repositories.

There should be no change in behavior with repositories that don't have preferred content configured. sync without --content won't populate them since that would be a surprising thing and a warning based transition was judged by the community to be not sufficient to avoid that possible large surprise. (See Changing sync to include content breaks UX)

A warning was added in August 2023 when it's run in a way that will change behavior. It would be good to wait until all git-annex users have gotten the version with the warning, and used it for a while, before finishing the transition.