Just a little nitpick…

Forward compatibility is not about applications being able to read documents created using an old superseeded version of the file format. That is actually what is called backward compatibility, quite common and relatively easy to enable… just keep the old handling code.

Forward compatibility is in fact the opposite. Old applications being able to read/handle documents created using a file format version that did not yet exist when the old application was developed. Which is less common because it is not so easy to do without limiting future possibilities. But if you do it right, it is a killer feature