Thank you, it is useful to know that NaNs are expected to be fully supported everywhere. It would be useful to add this to the VTK API reference, along with information of how VTK generally interprets NaN values when found in point coordinates, normals, point/cell arrays, image values, etc.
By searching in the code for presence of nan and isnan in implementation and tests, it seems that NaN is handled in IO classes, charting, lookup table related features, some basic statistics, and thresholding. This is a tiny portion of VTK, but probably covers the most common use cases. I would expect that most filters would behave quite randomly in presence of NaN values in input mesh point positions or in processed arrays, but so far we have stayed away from using NaN values, so I don’t have any problems to report.