Modster, who started UCLA's final two games in 2017 following Josh Rosen's injury, appeared in one game for the 0-3 Bruins this season but left after one play.
The 6-foot-1, 215-pound Modster redshirted in 2016 and appeared in five games last fall, passing for 671 yards with four touchdowns and no interceptions.
He competed for the starting job under new coach Chip Kelly, who went with Michigan graduate transfer Wilton Speight in the opener and then true freshman Dorian Thompson-Robinson after Speight suffered an injury.
Kelly said shortly before the season that Speight, Thompson-Robinson and Modster had distinguished themselves in a six-man competition.
"I would like to thank [former UCLA coach Jim Mora] for giving me the opportunity to play and attend such a prestigious school!" Modster wrote on Twitter. "Also, I'd like to thank the Bruin fans who continued to support my teammates an [sic] I throughout these years. I'd like to thank Coach Kelly and the strength coaches for allowing me to continue to play the game I love."
Modster's departure comes in the same week that Thompson-Robinson's father criticized Kelly in a series of tweets, writing Sunday that the Bruins coach is a "million dollar coach who bares [sic] no responsibility" for the team's winless start.