Today, setting liveRegion to true marks an iOS accessibility node with UIAccessibilityTraitUpdatesFrequently, which doesn't actually announce anything.

We could make the experience a bit closer to Android by sending an explicit announcement when a live region actually updates (we already have the information we'd need to do that in the bridge). It's not clear to me whether this would be a good idea or not though - unlike regular live region announcemnts (which in Flutter are always "polite"), it's not clear that announce would allow for interruptions from VoiceOver.

If we think this is the wrong way to go, we should probably provide better guidance than we do in the documentation beyond saying "this works in Android but not in iOS" - I would find it helpful to know what kind of approaches I should or should not consider taking (e.g., why or whether I should avoid doing SemanticsService.announce on iOS, or perhaps just why Flutter doesn't do that for me automatically on iOS).

/cc @goderbauer @jonahwilliams @darrenaustin @vick08 @caseyburkhardt @cbracken