What do you mean with "is the language open source"? The Ada specification is public [0] but not open source -- but the C and C++ specifications are not open source either, in the normal sense of the term. And like with C and C++, there are both open source and proprietary compilers for Ada, see e.g. [1]
What's mostly not open source (FAFAIK) is SPARK, the formal verification framework for Ada.
It's not a community project, AFAICT. Few people know how to build it from source.