There is a theory that teen runaway Tinya's actions in The Flash season 8 may have sent Iris to the Phantom Zone.  

The Flash season 8, episode 9, "Phantoms" introduced the character of Tinya (Mika Abdalla) as the Coast City Phantom 

The Arrowverse Tinya is based upon the comic book character of Tinya Wazoo, who was a member of the Legion of Super-Heroes that fought crime using the codenames Phantom Girl 

With Sue's contacts and Iris' talent for research, it did not take the two women long to find Tinya's birth mother, Renee Wazoo.  

The accident prompted Tinya's attack on Iris one episode later in The Flash season 8, episode 11, "Resurrection."  

When Sue demanded to know what Tinya had done, the teen threatened to do the same thing to Sue if she ever tried to find her again.  

Created by Superman co-creator Jerry Siegel and artist Jim Mooney, Tinya Wazoo was introduced as a native of the planet Bgztl 

Tinya's ability to enter and exit the Phantom Zone saved her and her friends in the Legion of Super-Heroes on more than one occasion.