We continue to see new patent applications for diffractive optics technology. We're all still waiting to see if the EF 600mm f/4 DO IS will become an actual consumer product, or if Canon is going to take things in a different direction.
From Canon News:
Canon describes how to manufacturer a diffractive optical element with a refractive resin component attached to it to further reduce chromatic aberrations.
From US Patent Application 20180143350:
in a refractive optical system using the refraction of light, lenses formed from glass materials having different dispersion characteristics are used together in combination to reduce the chromatic aberration. For example, in the case of an object lens of a telescope or the like, a glass material having low dispersion and a glass material having high dispersion are used in combination as a positive lens and a negative lens, respectively, to correct the chromatic aberration which appears on the axis.
The goal of diffractive optics lenses is to reduce the physical size of the lenses. This could help in a couple of areas such as smaller and lighter supertelephoto lenses and to reduce the size of longer fast lenses for full frame mirrorless applications.