And on the market, the idea is not new. The file level version exists in several other ecosystems. Jest has --changedSince, Nx does affected-test detection for monorepos, Bazel/Pants track build-graph edges. The Python space has pytest-testmon for local dev. Where the gap was(for me) is CI-shareable plus honest benchmarks. The CI-sharing part matters more than it sounds. If the map lives on one developer's machine, it does not help your PR pipeline.
Whether it reduces CD costs in practice depends a lot on how decoupled your test suite is. The honest range from my benchmark is ~21%(Flask, a tightly-coupled codebase) to ~96%(Boltons, one test file per module). Both are real per-commit replays, not just synthetic scenarios.