What? No, far from it. Bittorrent is pretty clean regarding illegal material (excluding piracy ofc) - which makes sense, after all you leak you ip address and the torrent that you are downloading to the whole network (if you have DHT enabled) and/or to your tracker, plus your ISP can see what you are sending and receiving (while there is a standard for encrypted transmission in bittorrent I do not think that it is widely used). What do you consider as "CSE" anyway? Would a picture of a girl at the beach be considered a "CSE"? If so you might find such torrents (I wouldn't know), but I think that calling it "CSE" is dishonest.
I'm a detective that works exclusively on online child abuse, I regularly arrest people who have downloaded and/or distributed IIOC over Bittorrent.
The definition of IIOC is provided by the Home Office and split into three categories. Ultimately it is decided by a jury although the categorisation is rarely contested.
Number of files shared that you know about, which I think is a pretty important distinction. Bit harder to track sharing over Tor, after all. (Which is the point!)
The majority is shared on normal social media and Bittorrent, although the worst is shared on dark web sites of the same robustness as OnionShare.