Chats is on for every organization, with pinned chats
Chats is now core functionality. The chat API, the Chats sidebar entry, search, the new-chat flow and chat threads work in every organization without a setting in Settings → Features, and the Experimental badge is gone. Voice keeps its own separate setting.
Chat threads can also be pinned: use the pin button in a thread header to keep it at the top of the Chats list and the sidebar.
See Web UI, Platform Chat and #3100.