In iOS, it's been a core part of the system frameworks for 9 years. That builds on over a decade of the same accessibility frameworks in macOS. There's really no reason that every new, popular, and widely-used user interface framework or library can't implement accessibility 100% on the two primary mobile operating systems.
Last year's WWDC session[1] on designing with accessibility in mind was just a reminder, but it really should be considered a mandatory part of user interface testing. If a company can't afford that aspect of testing, then they should really consider what frameworks and libraries they're using, how they're developing, and what trade-offs are involved.
Allow me to disagree. Mobile development is a mess these days. Doing stuff in React native is way easier than native (when there is a module that implements what you want), so most of the time you do things (a lot) faster. Sure, react native needs a lot of work and stop breaking things every single version they update and the modules need to step up in quality. I hope with the fabric rewrite, things get much better.