Are application-specific icons now accessible with QIcon::fromTheme? Think Konqueror favicons in bookmark menu.
Also, according to https://api.kde.org/frameworks/kbookmarks/html/kbookmarks-dependencies.html KBookmarks indirectly depends on KIconThemes via KTextWidgets anyway.
Tested Konqueror bookmarks. New bookmarks don't seem to get the favicon set until you explicitly trigger the "Update Favicon" action, afterwards they have it and it remains across restarts. So this seems to work as expected.
Regarding the indirect dependencies, yes, this does not actually change much on its own. I run into this while debugging icon loading issues in Okular on Android, which seems related to KIconThemes interfering, so less places where I need to strip it out help.