It seems like this was an opt-in feature, that was off by default, that allowed users to have a domain's favicon shown alongside the message. It doesn't seem like any contact info was sent. Indeed, if you go to the effort of turning on favicon showing, it also seems pretty obvious and intuitive the app will retrieve the favicon. And the way it's done here seems to be more privacy preserving than having a central server do this (and see what you request).

This feels a bit to me like Google saying "web browser leaks details of your requests to a third party" - alarming, until you realise the third party is the DNS server, and that's how the internet works.

This has some details and a code link: https://news.ycombinator.com/item?id=31434975

Thank you!