Not quite an answer to your question but slack engineering wrote a paper [1] on migrating their code base from PHP proper to Hack.

Specifically they used [2] 'partial mode' since "this loosens several restrictions to ease migration"

[1] https://slack.engineering/hacklang-at-slack-a-better-php/

[2] https://docs.hhvm.com/hack/source-code-fundamentals/program-...

EDIT: This would indicate that Hack may perhaps support standard PHP syntax but not the other way around.

EDIT: According to this [3] they are very compatible.

[3] https://www.oreilly.com/library/view/hack-and-hhvm/978149192...

They stopped supporting interoperability a while ago [0].

[0]: https://hhvm.com/blog/2018/09/12/end-of-php-support-future-o...

> Are there any large Hack codebases I can explore?

depends what you mean by 'large' - perhaps https://github.com/hhvm/user-documentation ?

> Does anyone know if FB tracks changes to PHP so Hack is "up to date"?

No, for the most part, Hack no longer considers PHP 'upstream'. Exceptions are things like security fixes to extension functions, if that particular extension function was derived from PHP.

Hack features have actually driven a lot of the additions into recent PHP versions. So the more relevant question might be the other way around