The question is: illegal content WHERE?
Illegal where the individual user is located. I don't think there is a technical solution because Bitcoin's scripting language is so open ended*. If it was impossible to store arbitrary data outside of OP_RETURN outputs and OP_RETURN outputs could be pruned (intra-transaction pruning) then it wouldn't be universally pruned. Local users would prune what they feel is objectionable or illegal on an individual basis. All that is academic though because it is so easy to encode arbitrary data in the blockchain in a manner which simply can not be pruned by full
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I don't think there is a technical solution either. I don't think there is a political one either given the immense number of jurisdictions. It is like trying to remove pee from a pool instantly.
If the "illegal where" is "where the individual user is located" there is really no solution since some data can not be pruned except some type of blacklist (a bad idea as above) where the most restrictive laws anywhere are enforced. That was kind of the point in asking the question. :-)