The historic red lines in the cold war were either one side reaching a first-strike position, or an unstoppable ground invasion in which both sides soldiers were directly involved and tactical nuclear weapons would usefully repel the invasion. Even then NATO troop deployments in Germany during the cold war reflect both a risk that this tipping point wouldn't trigger a nuclear war - as well as carefully calculated strategies to ensure that the risk of a nuclear escalation was high such as by stationing tactical nuclear weapons in Germany.

It's difficult to identify a similar red line in a likely conflict between any of the major nuclear powers.