We're currently using git flow to manage feature and hotfix branches in git, and satis to manage packages. Is there any way to tell satis to only list tagged branches as a release rather than listing every single branch that has been created?
Only tags are releases. Branches are no releases, they are moving targets, but they do exist.
You can try to create a release-only repo that does only contain your tags, but no branches, and grab the Satis metadata from there.
You could also run a cleanup script that removes any branches from the JSON data structure that you don't like.
You could simply ignore the branches, because they do not affect your workflow if you do use semantic versioning and require explicit version numbers instead of branches.