Answer
This isn't a missing setting to re-enable — it's a deliberate consequence of Apps10 using a fundamentally redesigned authentication mechanism compared to Apps9's. The old mechanism that proactively displayed a grace-period warning during normal use was removed as part of that redesign; instead, Apps10's login screen itself detects expired credentials at the moment of login and lets the user update their password directly from there, rather than warning them in advance. There's no configuration option to restore the old proactive-warning behavior — expired-credential handling in Apps10 happens entirely at login time instead.