It was just an idea. Groups still might be the best bet, since it’s the closest thing to a more formal micro ‘community,’ and that way tags/categories can be predesignated that they’re following.
From a UX perspective, there’s a reason those services cited above ask users to follow topics during the sign-up experience, and not after the fact, and that’s because engagement typically plummets after the initial experience.
So baking it into the sign-up experience is still very much needed!