Client and relay can implement some content filter feature.
For client, especially web clients, can use these libraries to implement the feature easily.
https://github.com/infinitered/nsfwjs
or
https://github.com/NsfwSpy/NsfwSpy.js
I think there are other libraries for iOS and Android that can be easily included in native clients.
I have already implemented in relay code of wss://nfrelay.app which gives filtering feature to its users.
