Criminology professor Buzz Hickey, played by Jonathan Banks, was a recurring character during season 5 of Community, but he didn't return for the final season of the show, so what happened to him? Actor Jonathan Banks is perhaps best known for his portrayal of Mike Ehrmantraut in Breaking Bad and its prequel/spin-off Better Call Saul, as well as El Camino: A Breaking Bad Movie, but he joined the cast of Community in 2014.
Season 5 was a time of change for Community. It marked the exit of series stars Chevy Chase and eventually Donald Glover, but also the return of series creator Dan Harmon, who was not involved with season 4 of the show. On screen, things had changed as well, like Jeff Winger (Joel McHale) returning to Greendale after graduating, now as a professor. He learned the ropes of how to take advantage of the position by his new officemate, Buzz Hickey. The criminology professor was a former police officer who only intended to teach at the community college for a short time, but ended up working there for fifteen years, eventually becoming part of Winger's former study group's orbit by joining the Save Greendale student-teacher alliance.
Appearing in 11 of the 13 episodes of Community season 5, Buzz became a pretty big part of the show, making up for the departure of several major characters, so it may seem a bit surprising that he did not return at all for season 6. There were several behind the scenes reasons why that may have happened, but Buzz's fate itself may have been revealed by an email. What actually happened wasn't made clear, but the season 6 episode "Basic Email Security" offered a major clue. When someone hacked the Greendale network and started releasing people's emails, a quick look at the lunch lady's inbox shows an email from Dean Pelton with the subject line "Buzz Hickey Memorial Services", indicating that Buzz is most likely deceased, and was simply killed off-screen by Community's writers.
Given how big a part of Community season 5 Buzz was, it's strange and a little sad that the writers may have decided to kill him off so unceremoniously, although there is an alternative. The email is "Buzz Hickey Memorial Services" plural, and it has been suggested by fans that Buzz simply retired from Greendale, launching a new venture which he's emailed Dean Pelton about, trying to get support for it. It's certainly more pleasant than the idea of him dying, at least.
As to the logistical reasons for Buzz's absence, it's seemingly more to do with Banks' schedule. After Community season 5 aired, NBC canceled the series, but only a month later it announced that the show would return for its sixth and final season on Yahoo! Screen, a short lived experiment focused on original content. However, right as season 5 of Community was beginning to air, it was confirmed that Jonathan Banks would reprise his role as Mike Ehrmantraut in the new series Better Call Saul, so while Community was in a position to have its final season, Banks was already committed to another show, and thus couldn't return as Buzz. Perhaps if the long-mooted Community movie ever happens, he could return (alive) for a final farewell.