This seems more like an appeal to emotion to justify the creation of censorship infrastructure than an actual solution to a problem.
I haven't heard of unsolicited CP being a common spam issue. And those who exchange CP intentionally can and do use encryption which makes such filters pointless.
Then you are not familiar with the Fediverse's patterns of establishing various levels of tolerance on different networks of instances for loli content (blah blah it's fiction blah blah) because otherwise it federates onto people's timelines and they're revolted. CWs do a lot to soften this.
The original argument seemed about illegal content. If you're just talking about unwanted porn, well, that's an argument for better local content filters, which is different from eradicating content at the source.
I haven't heard of unsolicited CP being a common spam issue. And those who exchange CP intentionally can and do use encryption which makes such filters pointless.