I may be wrong but to my knowledge stuff on bitcoin blockchain (up to ordinals attack) have been an indirect links to the illicit content not the content itself.
Unfortunately you're wrong: the RWTH Aachen Study mentioned in the Coindesk article from 2018 I linked above discovered at least one illegal picture in the blockchain. Of course, the methods used then to store data (it's possible this was even before OP_RETURN was introduced, although for pictures OP_RETURN isn't of any use due to its size limit) were much more sophisticated, using fake addresses and the like in large transactions that looked like "financial" ones, so there was no "explorer" one could view to see the contents.
[...] a widely-publicized report from RWTH Aachen University found one graphic image of child porn and 274 links to content depicting child abuse stored within the bitcoin blockchain.
SourceYeah, I skimmed through the PDF that was linked there and to be honest I wasn't convinced since there wasn't any proof provided (for understandable reasons of course) and the way they put it sounds like a very subjective matter.
I also specifically don't understand the bold part below. If they have extracted the file from the blockchain, how can it not be verified and why are they referring to an online forum in this case if the file is indeed on the blockchain and not a link to a content on another website.
The remaining instance is an image depicting mild nudity of a young woman.
In an online forum this image is claimed to show child pornography, albeit this claim cannot be verified (due to ethical concerns we refrain from providing a citation).