Stew meat can be overcooked in a slow cooker if the meat is left in the slow cooker longer than a few days at the high setting.
But as long as you turn the slow cooker down to the warm setting and keep the stew warm after cooking it then the stew meat should not overcook.
It's best to stop cooking the stew meat in the slow cooker after a day and turn it down to the warm setting so that it doesn't overcook.
Or turn the slow cooker off and then serve the stew and refrigerate the leftover stew in the slow cooker.
Then just reheat the stew again in the slow cooker and the slow cooker will then reheat the stew slowly without burning the stew meat.