While I still don’t know what happened to the ALT-click shortcut in 2.9 I have discovered how to accomplish the same thing via menus.
- Select all, and then make an object with the modifier you wish to toggle the active object, as shown in the video.
- Toggle the Viewport Display icon.
- Right-click and choose “Copy from Selected” from the resulting menu.
This copies the toggle status to any selected object with the modifier without applying or otherwise modifying data on objects that don’t.
