Michael Mann published his work in the late 1990's and had no clue of the craziness that would follow.
He used many sources of data which were obtained by just a simple phone call or email, there were on agreements about publishing their data as he had no reason to suspect there would ever be any need. These sources were a mix or private and public. Obtaining permission to publish from each of the source is simply not practical. There's also no need as newer work doesn't have this problem and has data that is freely available.
ie here
https://www.ncei.noaa.gov/cdo-web/https://climate-themetoffice.hub.arcgis.com/http://www.bom.gov.au/climate/data/and countless other sites.
The people that attack him for this have been well aware for decades now, but have a dishonest narrative to push.
