You could already subscribe to a post's comments on DEV, but the problem was many community members were not aware that you could. We had this feature pretty well hidden. From a post page, you had to click on the three dots to see a post's comment subscription options.
With the new comment subscription component, comment subscriptions are front and center in the discussion area of a post.
All the existing functionality still exists, it's just easier to find. 😄 You can subscribe to all comments, top comments, author comments or unsubscribe.
Take it for a spin to stay engaged with the posts that interest you.
Here are the PRs related to this feature for those interested.