Author here, that's a neat idea. However I also like the simplicity in having only one runtime interface. If the OS has to handle every version anyway, an app in the future could just use padding to feel like it is "clean".

  struct Context{
   padding: [u8;256], // old stuff
   ctx: ContextV42
  }

Just typing that felt a bit wrong though. On the other hand the app declaring what version it is feels like what executable formats (like elf) already solve. I am trying alternatives.