Warning: SPOILERS for The Flash Season 6, Episode 10 - "Marathon".
Cisco Ramon (Carlos Valdes) has a new mission on Earth-Prime, the amalgam world Oliver Queen (Stephen Amell) created as the Spectre when he rebirthed the Multiverse at the dawn of time. Earth Prime combined the original Earth-1 with Supergirl's Earth-38 and brought Black Lightning's home city of Freeland into the Arrowverse.
Naturally, Cisco took it upon himself to catalog the massive changes that have taken place since the Crisis. Earth-Prime is now geographically different from the Earth-1 Team Flash re and Cisco also realized their timeline and histories have been altered yet again in new and uncertain ways. Even more confusing, not only have many of Team Flash's old villains returned (some are even back from the dead) but there are also new villains on Earth-Prime they've never encountered before - such as this week's rogue, Dr. Kimiyo Hoshi, who is a new version of Doctor Light who works for the villainous Black Hole organization, i.e. the Big Bads of the second half of The Flash season 6. (The first Doctor Light the Flash fought in season 2 was the Linda Park of Earth-2.)
So, Cisco has been extremely busy sorting through the changes in Earth-Prime but this is also driven by his own sense of guilt because he feels he could have done more to prevent the Crisis if he hadn't surrendered his Vibe powers and taken the metahuman cure. Unsure over exactly how he feels about everything, Carlos Valdes is leaving The Flash.
In an interview with TVLine, The Flash's showrunner Eric Wallace confirmed that Carlos Valdes (and Cisco) are not exiting the superhero series: "Oh, no. No. [Cisco and Carlos] are not going anywhere,” Wallace said. “Cisco’s off on his journey to catalog the world, but he’ll be back very quickly." The Flash's head writer also promises that when Cisco returns from his sojourn, the answers he finds will shed light on "one of the biggest things Team Flash has ever dealt with", which promises the return of an old villain.
This isn't the first time there has been talk of Arrowverse fans will be what other new places, villains, and surprising changes Cisco will report he found throughout Earth-Prime when he returns to Central City on The Flash.