I just read through all the top-level replies to this post, and you’re the only one that actually understands this change. They didn’t just change the icon, they added new functionality.
Your description isn’t quite complete, though. Pressing it once adds to the playlist you most recently added to. Basically, it remembers which playlist you last added a song to, so if you’re listening to a radio station that matches one of your playlists vibes, it makes it really easy to add the songs as they play.
This new functionality perfectly matches my “flow” of music collection, since I add to separate playlists instead of to Liked. This feature changes nothing if you only ever add to Liked.
So basically, everyone in this post is complaining about a feature Spotify added that genuinely enhances my experience and is only a minor visual change for everyone else.