That device is called an Ankle Foot Orthosis (AFO).
More specifically, it looks like a posterior leaf spring AFO or a solid plastic AFO with straps.
What it does:
• Supports the ankle and foot
• Prevents foot drop
• Keeps the foot in a neutral position
• Used for stroke, nerve injury, cerebral palsy, or muscle weakness patients
It is commonly used in rehabilitation and orthopedic support.