Did you test your app with VoiceOver, the iOS screen reader for blind people? The difference between non-native and native navigation is particularly noticeable in that case, unless perhaps you fire a UIAccessibilityScreenChangedNotification after navigating to a new screen.
That's just the scenario I'm aware of. I haven't done any real projects with React Native, so there may be other pitfalls.
As passionate as I am about accessibility, I try not to use an accusing tone. There's no way we can know the state of mind, priorities, and constraints of the developer in question. Even large companies don't have unlimited resources. So we can't fault developers for looking for ways to do more with less. We just need to do what we can to make sure developers are informed about the pitfalls, and plead with them to consider important user experience factors like accessibility.
I think you mean 'built-in' into react native? Facebook routinely lets the community settle on the correct api aside from a limited core.
E.g. react router on web has gone through many versions and there are several community libraries for doing web navigation and how you want to track that application state.
And as a result there's a ton of churn, waiting for third parties to update their libraries (sometimes forever) and beginners having to make choices on how to do the most basic things without the experience or knowledge to help them make good decisions.
It's just beyond the needs of fb and what they cared to provide for. No one is paying for the extra dev work, so until some other entity pays or volunteers that will be the situation. Incidentally, Expo did extend RN to cover many of the missing APIs. It's just the reality we live in.