VTK specified data format should be documented within the code repository of VTK
VTK currently ouputs VTK Data Format 5.1 for Legacy and 2.2 for XML. There is no available documentation for these specific format.
The only file that can be found is here : https://vtk.org/wp-content/uploads/2015/04/file-formats.pdf and document Legacy 4.2 and no specific version for XML.
This document should be ported to markdown and integrated into the doxygen doc of VTK in order to be kept up to date easily.