I dont think defederation is the way to go here. It could be another scenario that could be solve elegantly via a tagging system.
But I agree that stuff which is in a legal grey area or outright forbidden in many countries should not show per default on All. There already is a NSFW setting that you can activate and deactivate (this btw can solve the issue to 99% for you right now if you’re willing to pass on other NSFW content).
There could be an additional setting to see things that “might be illegal in your country of residence” or simply NSFL. If you then mark such communities appropriatly it could solve the issue for people that want a) no exposure and b) no legal risk due to being shown such communities. And I count myself among those that dont want anything to do with loli or the likes.
Sometimes there is a man… Sometimes…