The settings schema now describes the key as "Deprecated: no longer has any effect. Subagent rows always use the default look." Nothing warns at startup. A configured command simply stops running, and users find out when their agent panel goes blank.
Without it, a session that runs several background agents is flying blind. Nothing in the default row shows which agent is burning context, which one is about to hit its window, which one went to the expensive model, or which one is silently re-paying for an expired cache.
People built on those additions. The docs still present the setting as live today: https://code.claude.com/docs/en/statusline (section on subagentStatusLine) and https://code.claude.com/docs/en/settings.
