And under all rules, the last time it happened was 1997, so it's still signficant

Actually not true. This is the first time ever these circuit breakers were triggered during regular trading hours. Even still, on election night, stock futures were only under the 'limit-down rule'. You could have still been buying futures while that restriction was in place, so this is very much different and unprecedented

Way before high frequency trading, to put some things in perspective.