This add-on is really awesome. I have installed it and it works very well on XF 2.2.13. Thank you so much to the author. All of your add-ons are really great.
You can also use this one: https://www.xendach.de/resources/cv6-buchstabenindex.440/
Its a litle bit fresher and works for me with actual Xenforo better, but... same as the one here from au lait - its also unsupported now. So bad. :-(