A release is legally required if a model is recognizable in any way. Even if you can’t see someone’s face, that person might be recognizable due to other contextual clues like tattoos, unique clothing, location, activity, or who they’re with.
In general, it’s good practice to have every model you work with sign a model release, no matter how much or little of the person is visible. This protects you and Stocksy from any potential problems. Additionally, some clients won’t buy unreleased images, even if it’s just hands and feet, so if you can get a release, get one.