This would be impractical for really large monorepos like the ones Google and Microsoft have. They have virtual file system layers on top (MS open sourced theirs) to prevent checking out the whole repo.

In fact, it’s not just useful for the CI/CD pipeline - any developers making significant changes to base libraries or core infrastructure should be able to use the VFS in combination with a system like Bazel to run all (or a significant sample of) affected tests across the company.