A patent showing how to improved flare control with DO elements has appeared. This goes along with some of the recent flare control patents we've seen for standard supertelephoto lenses.
This patent notes continued flare improvement for a 70-300 DO lens according to Canon News
An image pickup apparatus for the long wavelength band of 8~14μm uses light (hereinafter, the far referred to as long wavelength infrared region). Therefore, if the diffraction efficiency of the diffractive optical element with respect to a specific diffraction order is low in a part of the far-infrared region, diffracted light having a diffraction order unnecessary for imaging can be generated. This is not preferable because it causes flare.
While we don't believe we'll get an EF 60mm f/4 DO IS announcement in 2018, it's always possible another DO lens could be on the way.