Brushless motor design with additional separation layer at the air gap
Yes, these drive systems exist.

If the separation material is non-conductive, then the effect is no more than an additional airgap.

The effect of an increased airgap is to require more H field (longer or stronger magnets) to get the same B field in the gap, or conversely you get a lower B field for the same expenditure on magnets. This results in a different Kv, and a small reduction in efficiency.

If the separation material is conductive, then it will slow the rate of change of magnetic field due to circulating currents within it, limiting the maximum speed of the motor, and it will get hot, reducing efficiency. Stainless steel is the metal of choice, as it has a much higher resistivity than copper, and high alloy grades are essentially non-magnetic.