Technically they cannot but coaches can make it very difficult for you to keep it. For example, team meetings are mandatory and if you are 1 minute late a coach can close the door and claim you missed the meeting. They then cite you for violating team rules. Multiple violations no matter how minor are then used as justification for removal from the team. Players know when they are being pushed out even without it being said. If they survive all of that, at their end of the year meeting they are told, “no matter what you ever do, you will never play here so maybe you should hop in the portal and we will help you find another spot”. The coaches rarely help but the kid is better off leaving rather than risking being kicked off the team with a “violation of team rules” stigma. If the team kicks the kid off of the team for poor performance they have to honor the academic portion of the scholarship and they do not want to spend money on a kid not playing football.As far as I know they can't just pull a scholarship but they can tell you that you won't be getting any playing time. If you stay on you have to keep performing at practice or else you get the boot. When faced with that players transfer.
Each year I'd say probably 5-6 players leave before the portal era. It's that portal makes those numbers appear even more outsized
Or so I’ve heard.