A simplified chat UI and Adaptive Icons have hit Google's Allo chat app with version 22, which seems to be rolling out now to devices who have the application installed. That said, that doesn't necessarily mean that all users will have version 22 on their devices by the end of the day and the update could take a little while to reach everyone.
When it comes to the simplified chat UI, not a whole lot has changed and there's really nothing visually that's different from before. All that Google has really done here is make it so that there is no button you need to tap to start a group chat, which you would then have to start adding people after the fact if that's what you were looking to do, or search for who you wanted to solo chat with and then tap to start the chat. Now, you simply begin searching for users you want to chat with by entering the names or numbers in the box at the top, tap the contacts you want to add then hit the done button when you're ready. Allo will automatically start a Group Chat if there is more than one person in the list, otherwise, it will simply default to a solo chat.
In addition to this small change to the chat UI, there is also a new greeting page that pops up if you're a new Allo user or installing the application onto a new device. That's about it for new changes that are already present in this version of the app, but there are some things that look to be on the way with either the next version or within the next few versions. The first of these upcoming changes seems to be a couple of different camera effects for the camera that you can use to take selfie pictures inside of the app. One is called Bollywood zoom, and another is just a question mark sticker that can be enabled to place these in the frame of a photo when taken. Lastly, another upcoming feature it looks like Google is close to adding is the ability to transcribe audio messages and notes, though there is no indication of when these new changes will go live.