This rings a bell, the past illegal contents in the blockchain issue can be linked to that "
OP_Return" function.
I don't know how it works but based on your statement, it is possible to insert "
something" in to the blockchain.
By illegal contents, are you referring to how somone posted a code signature similar to that of a virus so that antivirus software attacks the blockchain.
It's therefore a good idea there's a 40byte limit or a bitcoin node attacking virus could be placed in the code (and that would be a very good strategy)...