I don't understand. Older versions of programs cannot open files created by new versions - that's a simple fact of life, has nothing to do with whether a program is open or not. I doubt many open programs would limit themselves in that way.

As for whether the MSCX format is documented or not, of course it is. MuseScore is fully open source. Every line of code responsible for reading and writing this format is freely available to you to examine, compile, etc. It doens't get more open than that.