Dimly lit (read: poorly lit) ice skating shows, typically at night with faint colored show lighting. ISO 12,800 needed to stop action at f2.8.
Although, I will say that on the 5D4, the difference between 6400 and 12,800 ISO isn't, IMO, dramatic enough to justify the extra weight/expense/restriction of an f2 lens vs a 2.8. (Note: this may NOT be the case for other cameras!) However, I would have considered anything slower than f2.8 to have been unusable in that situation.
To me, when we're talking about the difference between ISO 1600 and 3200, for instance, now f2 begins to sound more appealing because IQ begins to really drop off on most cameras above 1600...