I put a note about this in the RFC, but I can definitely say that React 19 won’t work today for many or most packages due to a few LegacyContext we’re working on deleting and mostly third-party packages that have legacy APIs. I hope we chip away at this enough that it becomes viable in the future.
It’s worth saying something given React 19 has gone stable that we’re aware that migration will be even more complex. We have no current plans with React 19, but this project has been a massive step in the right direction and we should all consider the APIs being removed in React 19 fully deprecated to make a future migration more simple.