The standalone /cost command has been removed and consolidated into /usage. The /usage command now has "cost" and "stats" as aliases, so typing /cost or /stats still works. The description changed from "Show plan usage limits" to "Show session cost, plan usage, and activity stats". For thin-client (remote) sessions, /usage now fetches the remote container's cost via a get_session_cost control request instead of displaying $0.00.
Evidence
Command definition with aliases (search for "Show session cost, plan usage, and activity stats") and remote cost fetching (search for "Loading remote cost").
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
One source agreesOne thing we can check says the same as this entry.
Anthropic's release notes agree
Merged /cost and /stats into /usage — both remain as typing shortcuts that open the relevant tab