The index gapping in RTH such a huge gap that you can not get out of the trade before whipping your account out has such a small chance (exactly 0.000367%) and only happens if you over leveraged yourself, so unless you are completely careless and you go all in all the time, you shouldn't worry about it.
Since TST requires a stop order with every trade, a "black swan" is probably a lowest level of concern for traders who have funded accounts. Using the AUTO OCO feature is a means of protection for a black swan, even with slippage, at least you'll get out with a specified loss.
When the ES opened "limit down" a few months back, it surely wiped out a lot of margin players, however those are overnight traders holding the prior session, and did not impact the day traders.
A stop is a market order that gets triggered by reaching a certain price, nothing more. If there's no market under you, you get a horrible fill. (See EUR/CHF earlier this year, where lots of uninformed traders thought they had "guaranteed stops" and didn't. Stops at 1.2000 filled at 1.0300 for a cool 1700-pip "slippage", etc.)