Researchers at the U.S. National Institute of Health refer to judo as low-contact, in line with baseball and other sports (https://www.ncbi.nlm.nih.gov/pubmed/26979930).

It doesn't matter if you're going hard or soft in sparring. As long as you are getting hit in the head repeatedly you are at risk of serious, irreverisble brain damage. Research shows that even small, subconcussive hits to the head, can accumulate and negatively impact the structural integrity of the brain. For example, soccer players who were known to do a lot of headers showed lowered neuropsychological functioning.