https://github.com/snapcore/snapd/pull/6767 adds the new icon set feature to snapd. During the review, it was discovered that neither the review-tools nor snapd were validating the input of desktop files using Icon= entries in general (outside of the new feature). Various checks were added to the review-tools with the intention of matching snapd's checks and flag when an Icon check would fail in snapd. While various unit and functional tests were added to the review-tools, the checks (and also the accompanying tests) for requiring .svg and .png extensions were too strict (ie, they did not match the new snapd checks) and should've only been applied to files in meta/gui/icons and not files in $SNAP. This had the unintentional effect of blocking valid icon extensions, such as .ico.
A fix in progress. While the fix to the review-tools is not yet in production, snap developers can either request a manual review for snap revisions that now incorrectly fail automated review and so someone can manually approve the revision (or change to use png or svg instead, but the update should be in production soon).
(switiching back to in-progess since the incomplete state as defined is completely wrong for this ...)