Git - jj[1] uses gitoxide for its git interop/backend (git being the backend of jj you're expected to use), both Apache/MIT; it functioned so cleanly I assumed it was invoking git directly, but apparently it's all custom!

For new things I'd guess PostgreSQL vs MySQL is probably on the same order as llvm vs gcc; probably with PostgreSQL being better off than llvm even.

[1]: https://github.com/jj-vcs/jj/ (started by a Googler as a hobby project, now their full-time job at Google, so kinda fits into the "big orgs will replace things with permissive-licensed versions" narrative)