As far as I could find out these images can’t even be read by older gimp versions. Keeping all this, you call it relevant data, could all of the time only be used by gimp itself. These are only needed, if you want to continue editing these images with gimp. Other programs could display the images but could not use the extra data.
I had a quick look in the settings and I think it’s possible to use the old format as default.
Edit : I just did a test and saved a small image using xcf. XnViewMP was able to display it without any problems. I can confirm that Okular is unable to open it. Gwenview is not installed on my system.
I doubt that. I’m sure it is ready. It’s not the gimp developers’ fault if other developers haven’t implemented that format yet in their programs.