I guess the React Native team used the tag because it’s one of the few non user-facing places in View to store custom data.
I don’t think uiautomator can find by tag so presumably their automated tests use something that can (e.g. Calabash).
Any thoughts for workarounds in Appium? I could add test ids via accessibilityLabel for testing and then remove them for production but that’s cumbersome and also kind of goes against the Appium ethos.