This is already done extensively: https://en.wikipedia.org/wiki/PhotoDNA and https://blog.cloudflare.com/the-csam-scanning-tool/
It worries me that anyone thinks it would be a good idea to have "fake news" and "inflammatory content" blocked at the device level. Obviously cloud providers can do whatever they want (though I doubt it catches any more than the lowest hanging fruit, encrypting then uploading would be uncatchable), but the idea that my device will have a list of disapproved content, and I'll have to appeal to the government to be allowed to view it in case of false positives? The day that becomes a reality freedom will truly be dead.