GIMP Version: 2.8.14
Operating System: Linux
GIMP Experience: Intermediate Level

List any ERROR messages you received:
XCF error: unsupported XCF File version 11 encountered.

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I found the answer to my question and thought I'd post what I found for others.

My Linux distribution default is gimp 2.8. I installed 2.10 appimage from https://github.com/aferrero2707/gimp-appimage/releases/.

I created a new file in 2.10, saved it as XCF, and opened it in 2.8. I got and error message, "unsupported xcf file version".

In 2.10, the Advanced Options: RGB color, 8-bit integer, Perceptual gamma (sRGB), Built-in RGB (GIMP built-in sRGB), Background Color, was not enough.

I finally figured out that I needed to also select Legacy in the drop-down on the Layers dialog.

When I saved the file the first time, the text read, "The image uses features from GIMP 2.10, disabling compression won't make the XCF fiel readable by older GIMP versions."

Now the text says, "Keep compression disable to make the XCF file readable by GIMP 2.6 and later."

I did a lot of searching. One link in the search list, https://www.gimp-forum.net/Thread-Gimp- ... Legacy-xcf, seemed to indicate if you didn't use the new features, that the XCF was compatible.

For the longest time I didn't see the drop-down next to the mode to select Legacy. It doesn't expand very wide. It's very narrow. See #1 and #2 in the image.

As I was putting the image together, I made the canvas size larger (Image > Canvas Size). I noticed that the chain on the left didn't clearly indicate if the Width and Height were separated. I clicked it several times and it didn't change. See #3. In Gimp 2.8, the link is clearly together or separated. I don't know if that is due to the Icon Theme I selected or not (Color).