On liked notebooks the title text still displays “Like this notebook”. I’d find it helpful if in this case the title text would match the action, namely unliking the notebook.
You’re basically required to memorize that the colored heart signifies “already liked”. Of course I can only speak for myself, but I don’t pay enough attention to that part of the UI to have learned the two states, and have on occasion unliked a notebook by accident.
Here’s a gif to illustrate the problem (the loop starts with the liked state):
This is fixed! Now when you’ve liked a notebook, the title text for the button changes to “Unlike this notebook”
(For what it’s worth, this talk.observablehq.com forum uses “like this post” / “undo like” as its mouseover text.)