So the answer is that the Qt folks treated the appearance of icons in menus as a bug on Mac OS and 'fixed' it in 6.7.3.

There is a sentence in the release notes:

https://code.qt.io/cgit/qt/qtreleasenotes.git/about/qt/6.7.3/release-note.md

• a2aa1f81a81 Determine Qt::AA_DontShowIconsInMenus default value based on platform The default value of Qt::AA_DontShowIconsInMenus is now determined based on the platform. On macOS icons will not show by default. To override, use QAction.iconVisibleInMenu for individual menu actions, or set Qt::AA_DontShowIconsInMenus to false.