I have some old branches in my git repository that are no longer under active development. I would like to archive the branches so that they don't show up by default when running git branch -l -r. I don't want to delete them, because I want to keep the history. How can I do this?

I know that it's possible to create a ref outside of refs/heads. For example, refs/archive/old_branch. Are there any consequences of doing that?

I believe the proper way to do this is to tag the branch. If you delete the branch after you have tagged it then you've effectively kept the branch around but it won't clutter your branch list.

If you need to go back to the branch just check out the tag. It will effectively restore the branch from the tag.

To archive and delete the branch:

git tag archive/<branchname> <branchname>
git branch -d <branchname>

To restore the branch some time later:

git checkout -b <branchname> archive/<branchname>

The history of the branch will be preserved exactly as it was when you tagged it.