This is actually already available to be implemented for clients with regard to NSFW content.

Amethyst, for instance, hides anything with the NSFW tag (and some other related tags) by default as sensitive content, and you can turn this off if you want it to appear in your feed again.

There is also a way to mark your own post as containing sensitive content in a way that SOME other clients recognize, using NIP-36, I believe. But most clients don't have this implemented.