It isn't "missing", as far as I know there was no accordion component in Ionic 1 either, however it's fairly simple to make one yourself, here are some links:
http://codepen.io/ionic/pen/uJkCz
https://forum.ionicframework.com/t/accordion-list/2832
http://loring-dodge.azurewebsites.net/ionic-item-expand
Or maybe the Ionic Market has something (haven't checked).