It's widely known there's child porn on the bitcoin blockchain, not familiar with any prosecutions that have happened.
Pretending that the law trumps all technology is as naive as pretending technology trumps all law. No extreme is true: you can build technological problems to solve many social or political problems (example: VeraCrypt with plausible denialability)
Are there actual bytes of child porn embedded in Bitcoin blockchain or only encrypted-or-obfuscated urls of possible websites that lead to child porn?
Example story from 2018 saying child porn bytes are not in the blockchain and non-tech journalists are misreporting it: https://news.bitcoin.com/no-isnt-child-porn-bitcoin-blockcha...
Any credible embedding of child porn bytes in the blockchain since that story?
Meaning would such attacks (attaching CSAM content to a blockchain) become more feasible at lower cost in the future?
Governments, under any law they wish to create. In some countries (eg Australia) they can (and have) created retrospective legislation that makes what was perfectly legal when you did it now illegal, and then prosecute you for it.