Mastodon is more like twitter, less like slack. Try a Jabber server, like prosody or ejabberd. There are web jabber clients, as well as native clients for Wind

Mastodon is more like twitter, less like slack. Try a Jabber server, like prosody or ejabberd. There are web jabber clients, as well as native clients for Windows, Mac, Linux, Android and iOS. You can require TLS on all connections, optionally log all conversations and chatrooms on the server, and easily federate servers to scale. Most Jabber servers have LDAP integration if you want to push user auth out that way. All of them can be configured to never talk to any other server outside of your control.