Same problem and no help from Sonos on this at all.  The live chat assured me this is a problem with my leads.....what rot!  This was all fine before the awful app was introduced.  Really regret purchasing Sonos goods now.  Awful experience.

It’s definitely disappeared a few updates ago. I had it and then it disappeared. They’re constantly updating the app lately so makes me think someone worked on that setting and forgot to add it back or something ‍♂️

It seems like the autoplay settings for the line-in source might have changed with the app update. If it's not showing in the settings, it could be an update issue. Checking with Sonos support or their latest release notes might help clarify if this feature is still available or if there's a new way to access it.

I can confirm that they mention the setting was lately added to Android in the changelogs. But comparing with my friends who have their up-to-date Android devices, the setting does show up, just not on iOS so there’s definitely a bug / forgotten issue here.