So basically PHP is still catching up to Hack and but not quite there yet?
You still can mix it with plain PHP, much like you can mix TypeScript with plain JavaScript.
I wish high-profile PHP projects, like Nextcloud, migrated to Hack eventually; it can be done piecemeal.
Have you used contexts/coeffects?
My understanding is that it's still yet to be fully rolled out to WWW, and nobody outside FB has yet played around with it. IMO it still has some DX issues (https://github.com/facebook/hhvm/issues/8828) and the examples given in documentation don't actually work in real life (https://github.com/hhvm/user-documentation/issues/1016).
It's definitely a fresh feature, iirc rolled out last winter.