Wikimedia sites have supported XCF files since at least 2006 (T3410). The idea was that the files would be used for collaboration on images and could be used to translate images. However, XCF files have not seen significant use, and continuing to support them no longer makes sense.
The XCF file format is described by https://gitlab.gnome.org/GNOME/gimp/-/blob/master/devel-docs/xcf.txt. That, however, is a descriptive specification, not a prescriptive one, and the files GIMP actually generates are not guaranteed to comply with it. That makes it more difficult for other tools to read and write XCF files. Major browsers do not support it, and there is limited support for it in other tools.
Wikimedia sites currently use (through Thumbor) the xcf2png utility from https://github.com/j-jorge/xcftools to convert XCF files to PNG files during thumbnailing. That library is largely unmaintained. It is based on a version of the spec from 2006, and received limited updates when it was forked in 2013 to make it compatible with GIMP 2.8. Updates since then have been maintenance only. The project also has two active unfixed vulnerabilities, CVE-2019-5086 (CVSS 7.5) and CVE-2019-5087 (CVSS 8.8). Thumbor has sandboxing in place to account for vulnerabilities in conversion libraries, but running a library with known vulnerabilities is less than ideal, to put it mildly.
ImageMagick also has limited support for XCF files. It is partially compatible with GIMP 2.10, but does not support all the features of the format. While some benefits would be gained by switching from xcftools to xcf2png (T260285), the inherent downsides of the format remain. (ed: XCF rendering on Wikimedia sites has used ImageMagick since 2023)
Because XCF files uploaded to Wikimedia sites are restricted to a limited feature set and can only be edited by a few tools, they are not very useful for collaboration. The application for translatable text has been entirely replaced by SVGs. Were XCFs being actively used for collaboration, I would expect to see them being frequently overwritten with new versions. https://quarry.wmflabs.org/query/47384, however, shows that almost all of the recent overwrites have been one user converting the files to a GIMP 2.8-compatible format.
Out of the 1,426 XCFs on Commons, only 640 are actually used anywhere: https://quarry.wmflabs.org/query/47385. That the list of top 5 most used XCF images includes a blank file is especially telling. Most of the XCFs with significant usage were used in talk page messages, not on content pages. The choice of XCF is not clear for almost all of them, and almost all would be better served by SVG or PNG format.
Proposed deprecation steps:
- Prevent new XCFs from being uploaded
- Prevent thumbnails from being displayed for XCFs
- Remove support for XCFs from Thumbor
The XCF files themselves would not be deleted, as some have some useful history that's worth retaining. MediaWiki itself could retain XCF support, it just wouldn't be enabled on Wikimedia sites.