Old methods of storing evil stuff required obfuscation: they would need to break it up into multiple chunks and reassembly would require specific software and knowledge of what the data is and how to reconstruct and interpret it exactly.

The old formats looked like this:

"Hi, I'm a Bitcoin transaction, here's my first output of 45 outputs - , here's my second output , here's my third output" along with a tonne of other stuff that has to get parsed out when processing the highly obfuscated material. This is thankfully also true of inscriptions.

OP_RETURN however is just a dump for raw, serialized data. It's not the same.

It says the equivalent of "Hi I'm a Bitcoin transaction, here's an unspendable output: end".

This wasn't a problem for tiny OP_RETURNs i.e their current limit of 80 bytes.

If they're permitted to be 100kb, that's where the abuse begins.

And that's the end of plausible deniability.

When the stuff gets processed - which it has to be for your node to verify that they are valid transactions - then you just have a raw, unadulterated file that will trigger primitive antivirus/forensics software to alert the user: "Hi, you have CP on your computer."

You now need a licence to run a Bitcoin node, everyone thinks you're disgusting if you do, and they're not even wrong.