The Pixel smartphones definitely get the best of Android when it comes to implementing a new feature or two, and this carries throughout the system UI including with Google Assistant.
In fact there are a number of Google Assistant features that have debuted on Pixel smartphones. Before making their way to other non-Pixel devices, Google typically tests a new feature(s) on its own Pixel lineup first. Then rolls it out fully to all Pixel phones before moving on to testing it on other popular Android devices.
In this case Google appears to now be testing the new Google Assistant design on phones that aren't the Pixel 4.
Google Assistant light bar is the new feature being tested
Google isn't just bringing the light bar to Assistant on non-Pixel devices. Rather, it's testing the new Assistant design in general.
The light bar is part of that though. And one of the features that makes Google Assistant more distinct with its new look. Keeping this in mind, not all of the Pixel 4's Google Assistant design is making its way to non-Pixel devices.
Based on some screenshots from 9To5Google, it looks to really be the light bar mostly. The design of Google Assistant's interface on the home screen when you activate it is changing it seems, but it won't be what you see on the Pixel 4 and Pixel 4 XL.
On those two devices, when you activate Google Assistant the backdrop is a sort of light or dark gradient that fades from a solid color to entirely transparent. It'll be light or dark depending on where you activate it.
For example, if you're activating it on the home screen it's generally white. But if you activate it somewhere the theme is in dark mode, it'll be dark. The design for non-pixel devices with the light bar just has a shorter back drop, but the entirety of its colors is still all solid. In this case white.
The design has yet to roll out more widely
So far the design has not rolled out to many devices. It's reported that there have been both Pixel and non-Pixel devices that have seen this new design though.
Which means Pixel 3 series and prior are likely to start seeing this new design with the light bar too at some point.
How long the testing will continue is unclear but tests for new UI changes can go for weeks or longer. So users shouldn't expect things to change when interacting with Google Assistant any time soon.
Google may also slip these changes under the radar during the rollout and forego any sort of formal announcement. So if you're eager to have things changed up a little bit, keep your eyes peeled for a new look the next time you ask Google to do something.