You're right: sorry. Not sure whence I invented that memory.
It has changed, but the only changes are that there must be a timing strip at 3km to identify who is in what group, that it can be extended with UCI permission to 5km, and that the incident must be independent of the riders control and, if a fall, must include several riders (that is the Roglic situation correction).
Weirdly, it applies therefore even if there is an HC mountain line a few hundred metres before a flatter finish.
It also specifies that to avail of the rule a rider mist have the "will of remaining with the riders in whose company he was riding at the moment of the incident": a clause I am happy to see ignored as it would prevent riders trying to assist those in trouble.