Please add Ctrl+Shift+S as shortcut for "Save as..."
I always use "Save as..." instead of "Save" to be able to go back to any previous version of my mscz files. I do the same when using GIMP and LibreOffice, and if disk space is not a problem, I guess that this is even the best thing one can do. It saved me lots of trouble and it is awesome to be able to go back even if the undo function is buggy (it is, in Musescore, for example when changing the measure) etc. - but I would like to use Ctrl+Shift+S for that. It's a common shortcut for "Save as...", and in MuseScore, it is currently unused, too. :-)
Comments
You can customize shortcuts however you like - see Edit / Preferences / Shortcuts. I don't think there is no OS-independent default that makes sense or it would probably already be done.
BTW, I've never done this, but it seems to me a better way of versioning could be to save your scores in Google Drive, which can version automatically. Or, if you don't mind the need to intervene manually (which you are already doing), you could save as MSCX (a text format) and use Git or some other version control system.