acf-collapse-all.js

// paste into WP console in admin
jQuery(document).ready(function($) {
	$('.acf-flexible-content .values .acf-icon.-collapse').click(function(e){
		$this = $(this);
		if(e.shiftKey) {
			open = $this.is('.-collapsed .-collapse');
			console.log('shift clicked');
			console.log('open: ', open);
			if (open){
				$all = $this.parentsUntil('.values').parent().find('.-collapsed .acf-icon.-collapse');
			} else {
				$all = $this.parentsUntil('.values').parent().find('.acf-icon.-collapse').not('.-collapsed .-collapse');
			}
			$all.not($this).click();
		}
	});
});

only tested on VERY simple use cases. Will expand to more later