As quoted here:
Looking Forward (Activated as of Feb 3!)
BasePart.Velocity and BasePart.RotVelocity are now deprecated (hidden from Properties window). There were multiple issues in their operation when dealing with a part and its assembly. For example, setting velocity on a part that isn’t a root part just doesn’t work, but it always reads the velocity of the specific part. The AssemblyLinearVelocity and AssemblyAngularVelocity properties, along with the new impulse functions, should provide all of the options necessary for setting specific velocities.
As for reading linear velocities of specific parts or positions, BasePart:GetVelocityAtPosition(Vector3 position) has been added which will return the specific velocity for any point on your assembly (more flexible than BasePart.Velocity).