It looks like HHVM is becoming an independent language, free from the shackles of PHP.
HHVM v3.30 will be the last release series where HHVM aims to support PHP.
The key dates are:
-- 2018-12-03: branch cut: expect PHP code to stop working with master and nightly builds after this date
-- 2018-12-17: expected release date for v3.30.0
-- 2019-01-28: expected release date for v4.0.0, without PHP support
-- 2019-11-19: expected end of support for v3.30
Ultimately, we recommend that projects either migrate entirely to the Hack language, or entirely to PHP7 and the PHP runtime.