Vantage can use her ult infinitely in mixtape.

Platform: PlayStation
Game Version: Apex Legends Season 27: Amped (probably even earlier…)
Game Mode: Mixtape

Normally, when Vantage has Sniper's Mark equipped, pressing the reload button will use the Ultimate Accelerator. 
In Mixtape, she has infinite Ultimate Accelerators and can do this over and over again.