Android is testing with floating bubble notifications for a remarkable time now, but there are no apps to use the feature in spite of an option available within the developer menu itself.
XDA had discovered that Google was testing the functionality of the new bubble functionality in its Messages app. Now, they have arranged to enable it. Executed in the app, the feature works very similar to that of Facebook Chat heads. When eventually enabled by Google, it will let you view complete conversations in addition to reply.
When Android Police first notified the notification bubbles in Android 10 beta 2, they were buggy and discouraging to use. At that time, there was a different way of viewing notifications, giving no additional functionality from the notification tab.
However, as the feature unfolded, it became clear that there was to be a higher degree of functionality that includes the ability to view whole conversations, as displayed. Google has said that it hopes developers will use this built-in bubbles API preferably, than any other self-made solutions, same as that of Facebook.
This functionality would bring a more eminent degree of cohesiveness to Android and will be decreasing the time of development by app developers who want to use such an interface.
As it is just an internal test conducted by Google, there is no information on when the feature will be launched in an official potential.
Google has been supporting developers to use the new API, so it is speculated that the feature is being developed for an official Android 11 release.
XDA has not shared what scheme they have used to test the new functionality, but it seemingly requires modifying the APK, making it questionable you will be able to replicate this on your own.
So, unless you are an exceptional developer, do not consider to play with it anytime soon.