Not exactly. A private repo is entirely private. You might have an open source project which has some deployment secrets etc that you want to check in to git. All the code and config is perfectly safe to expose to the internet, but you want to hide a specific file.
I think this pattern was common for things like Ansible and Terraform configs and dotfiles.
So a private submodule?
My point is, don't use an end-to-end encryption scheme if you want more than the ends decrypting it.
I believe git-secret[0] does what you describe. The author of Lunar[1] uses it to hide the private elements in a public repo.