That’s already the case. See eg.:

the Guardian

Child abuse imagery found within bitcoin

Researchers discover illegal content within the distributed ledger, making possession of it potentially unlawful in many countries

But I don’t think the Bitcoin blockchain is very well suited for this, so I don’t expect it to be widely used for this purpose. Not now, not when Bitcoin Core 30 is released, and certainly not if and when blocks fill up and fee pressure increases.