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.
Source