It varies by event. Personal pacers in the marathon are uncommon to begin with. In ultramarathons they are specifically not allowed until late in the race. That a track event would rule that way had become an unfortunate expectation.
Pacers in ultras largely serve a different purpose. They’re there for safety, especially once it gets dark. In events 50 miles and shorter, pacers usually aren’t allowed. Sure, the elite runners could get some actual pacing at the end but that’s a very small portion of pacing in ultras.
