In his book, Science and Secrets of Early Medicine, Jürgen Thorwald writes about the script of the Mohenjo-Daro and Harappa civilizations: (in rough translation)
Seals, bronze plaques and clay shells show that the inhabitants of the Indus state had their own script, which was a pictorial script, just the earliest scripts at the Nile and in Mesopotamia. (...) Undoubtedly, there used to exist a lot of writings created using this script. However, while Egyptians used papirus and the Mesopotamians used clay tablets for writing, the ancient Indians used cotton and perhaps also tree bark. No one knows it for sure, because the tropical climate probably destroyed everything that the former libraries and archives contained. Since all written documents were destroyed, our knowledge of the culture of the Indus valley civilization relies on archeological findings.
I was able to confirm that the Indus Valley Civilization had its own writing system and that it was used on seals and pottery. However, none of the resources I found said anything about using cotton or bark as the writing material.
Do we have any reasons to believe that the Indus Valley Civilization had extensive written documents and that they were lost due to the climate and the material used, as the author suggests?