I'm conflicted about this. Being able to use two versions of React on the same page has a very obvious practical benefit for developers, but man does it feel like bad practise. Yet more JS dependencies you're forcing users to download and parse before your site is usable.
I guess the overall argument with React and download/parse times is long since resolved (broadly, people don't care, those that do use Preact or similar) but for me personally it still feels like a downer for the industry at large.
As the post says, the React team isn't saying you _should_ do that. They're just trying to make it _possible_ to do so if you _need_ to.
One reason is that you can incrementally migrate parts of your app to React's upcoming Concurrent Mode if desired, while leaving the rest of an existing app in place.
I guess the overall argument with React and download/parse times is long since resolved (broadly, people don't care, those that do use Preact or similar) but for me personally it still feels like a downer for the industry at large.