SOLUTION: Disabling “Emulate 3 Button Mouse” in Preferences re-enables the ability to use the Copy to Selected alt-click shortcut.
I tried doing the select all, then alt-clicking the “display modifier in viewport” icon and it does nothing. I’m using 2.9 and actually using Kent’s file from the downloads since I wanted to skip ahead to the UV unwrapping bit.
Am I doing something wrong or was this behavior changed in 2.9?