The redesign is currently rolling out to some beta users
Google has been experimenting with various different approaches for attachments, emoji, gifs, and stickers in its SMS app in recent months. The latest incarnation is now being spotted in the beta version of Messages, replacing the blocky UI with more colorful and simplistic circular buttons.
As noted by 9to5Google, the new UI is made up of several round buttons with text underneath them, including gifs and stickers, even though it's possible they will also be accessible via the emoji picker too.
Google Messages is getting a streamlined new attachments UI was written by the awesome team at Android Police.