Most trolling and brigading is fairly low effort, and having a sharable collection of communities to troll makes it much easier to organize lots of people to do just that. Edit: yes external tools for that exist but having it built in makes it much easier to abuse and rope in people to do the trolling that otherwise would not bother.
But my main concern is that this isn’t opt-in for communities. Having a sharable and subscribable link with for example “meme” communities means that all of them are seeing a constant barrage of random people commenting that are not members of that community and do not have any idea what the community is about other than that it is somewhat loosely related to “memes”. It basically hides everything that is vital about the communities and actively damages attempts at community building.