File Format Specification
While MuseScore uses a xml-bases file format it can still be considered to be a proprietary file format, due to the fact that there is absolutely no specification about how this file format works. And this is bad. This means that anyone who wants to interact with MuseScore will have to reverse engineer the functionality, and it might change anytime.
This means that developing tools for MuseScore files is very hard to do. For example, someone might want to convert a musescore file to another format, or extract specific information. Or in the other direction, a program might want to pass music to musescore for engraving. One could have a LaTeX or OpenOffice Plugin for inserting scores into documents.
So I think MuseScore should start to keep an official specification of it’s file format.
Comments
I would say you have something backwards: you say that because it isn’t documented, it might change. The reality is the other way around: it’s undocumented precisely because it is constantly changing to meet changing requirements. Generally speaking, we have not wanted people writing tools that depend on this format, because there is an implied dependency on specifics that may we’ll change. Normally we would rather people working with MusicXML, or use the plug-in framework, or work directly with MuseScore source code. Maybe this will change going forward, but I suspect there are a lot of pieces that would need to come together.
Now of course every situation is different, and if you have some particular use case that isn’t served by these supported methods, feel free to describe it in more detail so it can be considered.
Beyond that, I would also observe the source code is the documentation, and it’s all open. So if someone wanted to take it upon themselves to produce something more specific to document the format, and promise to continue to update it through future changes, they are off e elections to do so! Could be interesting indeed. But probably not worth undertaking until MuseScore 4 is out, as it may well introduce significant changes.
Reverse engineering is quite straight forward because it's an XML format. I started doing this with v3 and now at 3.6.2 much of the structure is the same. If you want to write tools which interact with the basics you will probably be fine until v4.