I asked Marco to test it with a driver that supports modifiers, and he confirmed that it works.
I have also checked that kwin still works with drivers that don't support modifiers.
Test results before (6 failures):
The following tests FAILED:
30 - kwin-testLockScreen (Failed)
39 - kwin-testPointerInput (Failed)
57 - kwin-testSceneOpenGL-waylandonly (Failed)
71 - kwin-testKeyboardLayout (Failed)
74 - kwin-testKeymapCreationFailure-waylandonly (Failed)
88 - kwin-testColorCorrectNightColor-waylandonly (Failed)
Test results after (8 failures):
The following tests FAILED:
30 - kwin-testLockScreen (Failed)
39 - kwin-testPointerInput (Failed)
59 - kwin-testSceneOpenGLES-waylandonly (Failed)
60 - kwin-testNoXdgRuntimeDir (Failed)
61 - kwin-testNoXdgRuntimeDir-waylandonly (Failed)
71 - kwin-testKeyboardLayout (Failed)
72 - kwin-testKeyboardLayout-waylandonly (Failed)
74 - kwin-testKeymapCreationFailure-waylandonly (Failed)
I'm not sure what to make of this, but at least some of these failures are spurious.