So they changed where the buttons are. If you go into the Character Select Screen, click on a character, look toward the bottom, you will see options with hotkeys. On Keyboard/Mouse the default is S for Skip Campaign, click that and it will ask you ‘Are you sure you want to skip the expansion campaign? You cannot undo this on this character.’
I just tested this on my Eternal Characters, as I had already completed the campaign on my Seasonal character.
