Siddiq and Alden will be sticking around on The Walking Dead for awhile.
They each joined the show in Season 8. As Siddiq, Nash played an integral role as the man who Carl (Chandler Riggs) was saving when he got bit. He joined the community and vowed to contribute so that Carl's death was not in vein. His training as a doctor makes him very valuable, especially as Maggie's (Lauren Cohan) pregnancy progresses and her complications may come back.
As Al, McAuliffe showed that not all Saviors were bad, some of them were just trying to survive and joined up with a group who offered them protection. While being held as a prisoner at Hilltop, he defected and ended up developing a respectful relationship with Maggie. He's going to stick around and help build aqueducts and windmills and other medieval society necessities at Hilltop, and he may become a love interest for Maggie at some point, since he roughly corresponds to a comics character with whom Maggie has a relationship.
The Walking Dead lost two series regulars in Season 8, Riggs and Steven Ogg, who played Savior second-in-command Simon.
The Walking Dead Season 9 will premiere in the fall on AMC.