I am not aware of any current work to enable VolumeRendering in VisRTX and thus OptiX within VTK. There are some leanings toward promoting IndeX from ParaView up to VTK, but I don’t know of any plans for tight OptiX/IndeX interoperability.
OSPRay’s path tracer in VTK also lacks volume rendering. That option will get volume rendering later this year when we bump to OSPRay 2.x and add OpenVKL. OSPRay’s ray caster already has VolumeRendering, but could use minor tweaks to improve sampling in some situations.