/usr/include/Box2D/ is in libbox2d-dev 2.3.0+ds-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/Box2D/Box2D.h
/usr/include/Box2D/Collision/
/usr/include/Box2D/Collision/Shapes/
/usr/include/Box2D/Collision/Shapes/b2ChainShape.h
/usr/include/Box2D/Collision/Shapes/b2CircleShape.h
/usr/include/Box2D/Collision/Shapes/b2EdgeShape.h
/usr/include/Box2D/Collision/Shapes/b2PolygonShape.h
/usr/include/Box2D/Collision/Shapes/b2Shape.h
/usr/include/Box2D/Collision/b2BroadPhase.h
/usr/include/Box2D/Collision/b2Collision.h
/usr/include/Box2D/Collision/b2Distance.h
/usr/include/Box2D/Collision/b2DynamicTree.h
/usr/include/Box2D/Collision/b2TimeOfImpact.h
/usr/include/Box2D/Common/
/usr/include/Box2D/Common/b2BlockAllocator.h
/usr/include/Box2D/Common/b2Draw.h
/usr/include/Box2D/Common/b2GrowableStack.h
/usr/include/Box2D/Common/b2Math.h
/usr/include/Box2D/Common/b2Settings.h
/usr/include/Box2D/Common/b2StackAllocator.h
/usr/include/Box2D/Common/b2Timer.h
/usr/include/Box2D/Dynamics/
/usr/include/Box2D/Dynamics/Contacts/
/usr/include/Box2D/Dynamics/Contacts/b2ChainAndCircleContact.h
/usr/include/Box2D/Dynamics/Contacts/b2ChainAndPolygonContact.h
/usr/include/Box2D/Dynamics/Contacts/b2CircleContact.h
/usr/include/Box2D/Dynamics/Contacts/b2Contact.h
/usr/include/Box2D/Dynamics/Contacts/b2ContactSolver.h
/usr/include/Box2D/Dynamics/Contacts/b2EdgeAndCircleContact.h
/usr/include/Box2D/Dynamics/Contacts/b2EdgeAndPolygonContact.h
/usr/include/Box2D/Dynamics/Contacts/b2PolygonAndCircleContact.h
/usr/include/Box2D/Dynamics/Contacts/b2PolygonContact.h
/usr/include/Box2D/Dynamics/Joints/
/usr/include/Box2D/Dynamics/Joints/b2DistanceJoint.h
/usr/include/Box2D/Dynamics/Joints/b2FrictionJoint.h
/usr/include/Box2D/Dynamics/Joints/b2GearJoint.h
/usr/include/Box2D/Dynamics/Joints/b2Joint.h
/usr/include/Box2D/Dynamics/Joints/b2MotorJoint.h
/usr/include/Box2D/Dynamics/Joints/b2MouseJoint.h
/usr/include/Box2D/Dynamics/Joints/b2PrismaticJoint.h
/usr/include/Box2D/Dynamics/Joints/b2PulleyJoint.h
/usr/include/Box2D/Dynamics/Joints/b2RevoluteJoint.h
/usr/include/Box2D/Dynamics/Joints/b2RopeJoint.h
/usr/include/Box2D/Dynamics/Joints/b2WeldJoint.h
/usr/include/Box2D/Dynamics/Joints/b2WheelJoint.h
/usr/include/Box2D/Dynamics/b2Body.h
/usr/include/Box2D/Dynamics/b2ContactManager.h
/usr/include/Box2D/Dynamics/b2Fixture.h
/usr/include/Box2D/Dynamics/b2Island.h
/usr/include/Box2D/Dynamics/b2TimeStep.h
/usr/include/Box2D/Dynamics/b2World.h
/usr/include/Box2D/Dynamics/b2WorldCallbacks.h
/usr/include/Box2D/Rope/
/usr/include/Box2D/Rope/b2Rope.h