I found it here.

Here is the (IMHO) relevant part (emphasis mine):

If you use the App with an account hosted on a third-party server, then there are only a few cases where FUTO may learn any information at all about your use of the App.

• If you have the App configured to use Google push notifications, then FUTO’s push notification gateway will be involved in delivering push notifications from Google to your device. Push notifications include information on the Matrix event that generated the push, including: the human-readable name and Matrix user identifier for the user who sent the event and the Matrix room identifier for the room containing the event. Users who desire a greater level of privacy can configure the App to use UnifiedPush instead of Google Cloud Messaging. When a user on a third-party server receives a UnifiedPush notification through their third-party push server, FUTO does not see the notification or learn anything about it.

• If you obtained the App from the Google Play Store, and the App crashes, then we will receive a Firebase crash report.

• If you sign up for our mailing list in the app, then we learn your email address.

The notification should be UnifiedPush by default, and crash debug reports should be opt-in, not automatically sent out to the dev via Google… (I don’t think it is even possible to opt out)