What do you mean by "optional"?. You are including it [2] [3] [4] as one of the options (not re-implementation of the original paper [1]) and it is one of the dependencies [4] (Prophet [5] is an open-source library built by facebook research).
I think we should distinguish between science/open-source and policies when we mention open-source projects.
*References*: [1]: https://facebook.github.io/prophet/#:~:text=Prophet%20is%20a....
[2]: https://github.com/unit8co/darts/blob/master/darts/models/pr...
[3]: https://github.com/unit8co/darts/blob/master/setup.py#L12
[4]: https://github.com/unit8co/darts/blob/master/requirements/pr...