Nothing has changed for constraining proportion in InDesign 2021.
It sounds like you may be having another application or extension which capturing your keystrokes so modifier keys don't work properly. To test, reboot your computer. If possible, disable startup applications. OPEN INDESIGN FIRST and test the shortcut. Chances are it will work fine. Then watch carefully as you launch other applications. Often virus blockers or browser extensions capture keystrokes and you may to change their settings.
Sorry to hear about this. I completely agree with @Steve Werner. In addition to what he has suggested, you may also try starting your computer in safe mode and work on the app there. Let us know if you have similar issues with keyboard shortcuts there. Looking forward to your response.