Unfortunately, Anki cannot detect if the Shift button is being held while starting on all platforms. In particular, it doesn’t work on Wayland, probably because Wayland handles input events in a much stricter way than X11, or other equivalent X servers: by default, an application can only be notified of keystrokes that happened while the application was focused, probably for a security reason.
Anyways, besides that, on my computer Anki starts too quickly to even leave me a chance to hold Shift, and I can’t hold it before launching it because it is bound on a shortcut, and holding Shift modifies that shortcut.
This is to say: it would be neat if there was at least an other way to start Anki without syncing and without add-ons. For instance, a certain command line argument could inhibit add-on loading, similarly to the
--safe-mode of firefox.