I am sure it is already there. You are naive if you think that garbage isn’t already on the chain

As a node runner I was worried about this too. Turns out that illegal content has been present on the blockchain for a few years now:

https://www.theguardian.com/technology/2018/mar/20/child-abuse-imagery-bitcoin-blockchain-illegal-content

We probably don't see this sort of content too often because nobody wants to risk getting caught broadcasting it.