It is optional in the sense that D's GC is also optional. Technically true but you have to go out of the way to make it work for you, average libraries off the shelf cannot be easily utilized.
Nim now has ARC/ORC, which is a reference counting scheme similar to Swift. This is not a "generational GC" like Java, Go, or D.
The entire standard library and most average libraries, "just work" with it, and it will be the default near future.