I am not sure when Sonos will bring back the alphabet  sidebar, but you can use the search function for songs, artists and albums in your library right now.

Select Music Library from Your Sources. You will see “Search Music Library” at the bottom of the screen just above the room you have selected.

I have Safari, Web server does not have a search function for Music Library.  that I can find? Only searches services?

IOS on iPhone app also does not have search for Music Library?

Only search function I have is on my laptop which has not been updated, old app works great!

It’s on all of my iOS devices and on my Mac app. You may need to reset your mobile app.