This is a proposal by people funded by companies that would provide the services for this (https://balkaninsight.com/2023/09/25/who-benefits-inside-the-eus-fight-over-scanning-for-child-sex-content ).
A lot of actual politicians oppose this https://tbbacherle.eu/2024/06/18/open-letter/
IIRC Mono was mostly used for WASM as it was optimized for smaller builds than the full fat CoreCLR (talking about .NET non-Framework Mono)