I'm 100% comfortable with serialization and various file formats including XML and others.
What I'm mostly interested in is the variance in platforms regarding where to save data. I'm mostly interested in hearing from people who have shipped MonoGame games on multiple platforms.
If I don't hear anything more authoritative, I will start with
var appDataDir = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData);
appDataDir = Path.Combine(appDataDir, "/:MyGameName:\\");
Directory.CreateDirectory(appDataDir); // to-do: handle exceptions
var settingsFileName = Path.Combine(appDataDir, "Settings.ini");
And use it until it I run into an issue. I'm sure this will work find on Windows and probably Mac. Don't know about other platforms like mobile and consoles.