Element (the primary Matrix software) definitely has Slack and Discord in its sights.
I don't think there are any serious "self-hosted Slack-like" contenders that are XMPP-based right now. You can piece components together (yay, standards!) and I did exactly this for the IETF's XMPP deployment recently. But it's far from being a cohesive easy-to-deploy product. Simply because nobody is building that right now. It takes time and resources and there's no money in it.[1]
People who do set out to build Slack clones (projects like Mattermost and Rocket Chat) and earn money don't have features such as federation on their priority list and don't build on top of Matrix/XMPP. They roll their own custom protocols and as far as I can see they are fairly content with that decision.
[1] There's even less money it, but nevertheless I am currently working on such a self-hostable "package" for XMPP. However rather than focusing on the team chat use-case (Slack/etc.) I'm focusing on personal messaging (WhatsApp/etc.): https://snikket.org/ if you're interested. It's possible I will broaden the scope one day.
EDIT: typo