Slack has a few eng blog posts from last year which seem to indicate they still use Hack, among other languages.

https://slack.engineering/hakana-taking-hack-seriously/ describes their open source enhanced Hack type checker, which is still actively maintained (most recent commit yesterday).

And https://slack.engineering/slacks-migration-to-a-cellular-arc... mentions they use a mix of languages: "services in the user-facing request path are written in Hack, Go, Java, and C++"