SMAPI used to support lyric display, although I never saw a music service that actually implemented it. However in the latest SMAPI docs there is no mention of it, so I am guessing the ability was removed from the ‘updated’ mobile app.

TL;DR: never.

Depending on what you are expecting to see lyrics on, TVONOS supports Lyric display - but you’ll need an Android TV / Firestick or Android Tablet

(Not sure if I am allowed to post links - but should be easy for you to find)

+1, Would be nice indeed. Using Sonos for Android.

AirPlay or Sonos Connect from Apple Music or Spotify (on other music apps), then you can see the lyrics on either of those apps. No other option. 

The Sonos app is just a controller, getting the album art and music quality metadata from the streaming service (via the speaker?), but the app itself is not playing the track so that probably makes lyrics integration tricky.