I do agree, this would be great. But Deezer told us that this isn’t happening any time soon :( 

A few years back I evaluated Deezer as an option to Spotify and my single most needed feature was something similar to Spotify Connect, especially on Sonos. (i.e. “cast” and control music playback on Sonos from the Deezer app). Now more than ever I want to move away from Spotify but as far as I understand, Deezer have still not added this feature? As a Deezer/Sonos user on Android I will have to use the Sonos app? Is that correct? With the current app-updates to Sonos this is an even worse option today than a few years back ;)

(Added Android for clarity, airplay is not an option)

Yes, you will need to use the Sonos app, and/or Sonos Voice Control if you are in a supported region.

I feel like the issues with the Sonos app we experienced last year are behind us. It is functioning smoothly for me.

To me it just not only about “issues” with the app, the app is poorly designed and lacks features. I became very aware of this recently when the Spotify Connect integration broke for several users, including me, and instead of being able to control my music using the Spotify app I was forced to use the Sonos app for many weeks. This will unfortunately be a deal breaker to me.