Files should be saved in maximally-compatible format versions where possible.

[ghost]

GIMP versions lower than 2.10 can't load new XCF files (XCF version 11) unless the user specifies them to be saved in legacy mode, but this isn't the default option and has to be manually set upon saving. Ideally all XCF files in the repository should be re-saved in a maximally-compatible version of the format in cases where they don't actually use any features that would require new versions of the software.

The same may also be true for BLEND files between versions 2.79 & 2.80, but as far as I know this is a much more intractible incompatibility that doesn't just come with an option within Blender to save the file in a legacy-compatible format.