The programming language described by standard ECMA-334 is, like most any other ECMA standard, non-proprietary, open for anyone to use, and not particularly tied to any one platform.
There is of course a particular implementation of that language which is proprietary, closed source, and tied to a particular platform. However, this is not a particularly unique feature of the language. The same is also true of ECMA-262, ISO-1539, ISO-9899, ISO-10279, and many others.
For its part, Mono is supported by the same people that design the Mono branch of the language/APIs. This is a quite robust fork of the platform, which includes a great many useful APIs which are specific to that version, and forms the foundation of popular tools such as the Unity game development framework.
There is of course a particular implementation of that language which is proprietary, closed source, and tied to a particular platform. However, this is not a particularly unique feature of the language. The same is also true of ECMA-262, ISO-1539, ISO-9899, ISO-10279, and many others.
For its part, Mono is supported by the same people that design the Mono branch of the language/APIs. This is a quite robust fork of the platform, which includes a great many useful APIs which are specific to that version, and forms the foundation of popular tools such as the Unity game development framework.