Update: Per Deadline, Keiynan will not be returning to Legends of Tomorrow or The Flash as a series regular but the door is open for guest returns such as his appearance in the 5th season premiere of The Flash. There is literally no information on why he isn’t returning as series regulars to the respective shows.
The actor who plays the superhero on The Flash and Legends of Tomorrow will not be returning to either show as a series regular, Deadline has confirmed.
Per TVLine Keiynan Lonsdale is not returning to Legends of Tomorrow for the upcoming 4th season. No details are mentioned as to why he’s leaving but TVLine does mention he’ll return for The Flash’s 5th season premiere.