I use the ever living hell out of .git/info/exclude. Works great for scripts/Makefiles I only want locally and collaborators wouldn’t care about or be able to use.
git-home/
company-project/ <-- git repo with main checked out
ephemera/ <-- my private repo
my-data-script.py
work/ <-- gitignored
company-project-feature-X/ <-- worktree on feature-X branch
company-project-feature-Y/ <-- worktree on feature-Y branch
This way, too, I can easily use the same ephemera scripts across multiple branches, or even multiple repos, concurrently.