Why is foster still on the roster? He’s hasn't played a game in 2.5 years since getting him...
Because he'd be the best player on the defense if he was healthy and there's no harm in letting him try to get his athleticism back. Chase Young will hopefully be better than him but he would be a force at ILB if healthy. Look at Jaylon Smith in Dallas: horrific knee injury at the end of 2015, drafted in 2016 by Dallas, spent all of 2016 on IR, and came back slowly in the 2017 season before becoming a stud and signing a 5 year, $64M extension in 2018.
Foster hurt himself last May so it's been ~15 months since his injury. Considering some guys need to be inactive on game day anyway and he's got a ton of potential I have no problem burning a spot for him to see if he can get his game back. He's only 26 and could be a fixture at LB for a while if he gets it back.